It sounds to me as if what you need to do is to have a frank conversation with your partner about things. Sexual attraction for a partner can ebb and flow, and one option might be that some good communication would improve things between the two of you on that front. Alternatively, you could stay together in an "open" relationship, where the value of your partnership can be preserved but not at the cost of your sexuality. The point is that between the two of you, communicating well about what you have and what you (now) lack, there might be some creative problem-solving that would give a more optimal result than the options you are currently considering.
There is an old saying (I'm told it originates with Kant, but I am not sure about that), which goes, "'ought' implies 'can.'" The idea is that you can only be held responsible or have an obligation to do something (you "ought" to do it) if it is something that is under your control. Do you suppose that emotions (such as love) are under one's voluntary control? I'm inclined to doubt that (with a few reservations, which I will get to momently). But if love is not something that you can voluntarily control, then it makes no sense to say that you have an obligation to love your mother (or anyone else, for that matter).
On the other hand, we do also evaluate people on the basis of how they feel about things, and on the basis of emotions they have and display. We same that some anger, for example, is inappropriate, and we regard most examples of hatred as at least unfortunate, if not contemptible. Does this make sense? I think it does make some sense, in that at least one of the things we value (positively or negatively) in people are their characters, and this includes emotions and such. So does this violate "ought" implies "can"? That seems to me to be more complicated, because while it does seem implausible to say that we can turn emotions on and off like faucets, it also seems plausible to say that the sorts of characteristics we have are at least to some extent the result of things we are able to do--for example, we can train ourselves to improve upon the way we might react to certain things (go to anger management therapy, for example, or biting our tongues when we feel impulses to say things that are cruel or hateful). people who have (or display) bad character may not be able to act any differently at the moment, but it still makes sense for us to hold them responsible for not having done the character-building that would have made them better people who would not have behaved so badly in that moment.
So back to loving your mother. On the one hand, if what you find unlovable about her are her values, it could well be that your reaction is simply the right one. There may be some respect(s) in which one need to respect and recognize the special relationship of parent and child, but I can't see why someone who is a bad person deserves to be loved by anyone, child or otherwise. On the other hand, before you take this as an excuse, you might do some serious double checking on your own values, which are leading you to reject hers. All I can know for sure, if yours and hers conflict, is that at least one of you is wrong. It might not be her, and it also might be both of you!
I suppose the ideal is that love between a parent and a child is sustained quite naturally, and is actually deserved in both cases. But maybe that is not going to be possible in this case. If so, then as I said, there are still some reasonable constraints about how you should respond to your mother (because she is your mother), having to do with civility and respect for her role in your life. There are certain duties and kinds of loyalties that we reasonably expect along these lines (though as I have already indicated, these are defeasible, if those to whom we normally would supply these have violated the ground for such things badly enough--an abusive parent, for example, may reasonably be thought to have lost any claim even on the most basic forms of loyalty from the child he or she has abused). At the very, very least, your mother deserves from you the kind of civil and polite responses you would provide as a matter of common decency.
But I can't help but wonder if perhaps you can do better than this. I don't know your mother or her values that you object to. But I suspect that civil discussion and allowing her to explain those values to you, and why they are important to her might at least allow you to achieve a level of understanding that would allow you to be more tolerant. Tolerance is not the same as love, I agree, but it's a start!
I rather expect that monogamous marriage is more the result of the agricultural revolution than an artifact of our life spans. It was when we changed from hunter-gatherers to "property-owners" (so that we could raise our crops and lay claim to the fruits of our agricultural labors), I suspect, that someone got the brilliant idea that we could own not only land and what it produces, but also start building private homes (since farmers don't need to keep moving around to hunt and gather) and having private families. That was when we began to "own" other people--including slaves, spouses, and children. In many early versions, this "ownership" was strictly one-way: it was not adultery (or a violation of marriage) for a man to have sex with some woman who was not owned by someone else (either as wife or daughter, for example), but was a violation of marriage if the wife engaged in such extra-marital activity.
The institution has changed as human society has changed, but I still think it is an artifact of another age in this sense. That does not mean it does not still have value. On the other hand, it does strike me as quite plausible to think that we would not have this institution if our average life span was only fifteen years. But then I think other problems would arise (e.g. for survival of our species).
You might extend your speculations in other interesting ways, by the way: What if the average life expectancy of only females was 15, but for males it was about what it is now in the US, for example? Or what if it was the other way around, with males being very short-lived and females having 80 years? But anyway, I agree with the thrust of your question, which is that there is no "natural necessity" in this social arrangement, as we know it.
Some of the stereotypes that drive such preferences could, of course, be racist. But it is also true that the factors that attract us to others erotically are not generally matters of simple choice, and the mere presence of a preference "type" does not seem to me to be clear evidence of racism. Some of us prefer tall partners--is this "shortism" because we tend not to prefer short partners?
I think of racism as consisting in beliefs or practices that would deny equal moral, political, or economic rights to members of the targeted race. I don't think anyone has any kind of a right to have me attracted to them as a potential romantic or sex partner, so I can't really see how my preferences in these areas can have the consequences of denying anyone equal rights.
Having said this, I also think that many cultures do lend some support to sexism or to regarding women as second-class citizens. I wouldn't blame a woman from such a culture for having a preference against men of that culture.
I will leave it to others to supply whatever they may think is a good reason for supposing that there is some kind of rule written in Heaven (about which, more in a moment!) as to why "one size fits all" in terms of fulfilling sexual relationships. As you quite rightly point out, it is one thing not to abjure any kind of relationship that amounts to abuse or coersion, and quite another to lump in with these any sort of relationship that deviates from the social norm of a single partner.
Nor can it even be said that single-partner relationships are a norm that is or has been always realized in human societies, even if it is endorsed in most (but not all) cultures. Were that the case, prostitution would not be, as the saying goes, the world's oldest profession, and polygamy would be unknown.
I rather suspect that the historical basis for the very restrictive ideal to which you refer goes back to a time when women were regarded as men's property, which is why in so many cultures the sexual fidelity of the female has been a topic of acute and intense social and legal mandates, whereas the sexual fidelity of males has been treated as a matter of indifference (or even regarded as a kind of "unmanly" aberration). This "double standard" received strong support from religious institutions, which have generally regarded sex purely instrumentally as the process required for procreation, but the pleasures of which were mostly regarded as morally corrosive.
Some will object that there are strong evolutionary advantages that accrue to societies in which both parents are deeply invested in child-raising. But, of course, these advantages speak not at all to the issue of polyamorous relationships. (Must a "swinger" not be deeply invested in her offspring? Must a society in which polyamourous relationships are widely accepted also be one in which children are too often neglected? I fail to see why! Of course, if the only way in which adequate support of children is expedited in a culture is though strong enforcement of norms of private families, then of course those who feel unfulfilled in such relationships will be forced into decisions that can have the cionsequence of putting the welfare of children at risk.) Moreover, these same considerations apply not at all to relationships not involving fertility (either from controlled fertility, infertility, or in same sex relationships not involving adoption).
Not all people prefer polyamorous relationships, of course. But I can think of no good reason why such relationships among fully consenting adults should be anathematized or demonized in the ways they often are. So I stand with you in hoping someone (else) can explain well why such relationships are regarded so negatively.
I'm tempted to say that when it comes to love, all is mystery, and leave it at that. But that would be a little too quick, perhaps.
You ask several questions, so I will try to reply to them one-by-one.
(1) If you show all signs of loving someone, you probably do. However, we make a distinction between infatuation and real love, and so the real test of which of these it is will be a matter of time. Infatuation dies pretty quickly, whereas love is more durable.
(2) A good part of love probably really is "just in your head," or, more likely, in your biochemistry more broadly. When we are engaged in this way, there are very significant differences in cognition, sensation, emotion, and all of the neurological and endocrinological systems towhich these are related. It certainly isn't love if it doesn't change you in lots of ways!
(3) You also want to know what it is about your beloved that brings out these reactions in you. The answer seems to be that it is lots of things. But there is a risk of cart-before-the-horse here. It is probably true that how someone looks sometimes plays an important original role in the process of falling in love. But it is also true that falling in love plays an important role in how someone looks to you. Here's an autobiographical example from my own past. When I was in high school, I started dating a very nice girl who had a very large chip our of one of her inciser teeth. At first, I found this unfortunate--the only obvious flaw in my fair beloved! But then an oddd thing happened--I came to love that chipped tooth, and when she told me her parents were taking her in to have it capped, I was very dissappointed! I was going to miss that little irregularity!
(4) The same can be said (cart-and-horse) about peersonality traits. In some cases, these serve as initial attractants, wwhereas in other cases, these are matters for reassessment after the relationship has already gotten going.
(5) The right reasons for loving someone have to do with the sorts of things that create the possibility for personal growth--both at the sexual level and also at the emotional and intellectual levels. The wrong reasons, very roughly, are those that create the possibility of personal deterioration. We tend to look for those who share our same values, but this, too, can be a cart-and-horse matter: Those we love can help us to change our values. Those we should love would help us to change our values for the better; and those we should not love would help us change our values for the worse.
(6) Apart from the issue of values, we do well to love those with whom we are compatible, sexually, emotionally, and intellectually. The most dangerous kinds of love-affairs are those with toxic inconsistencies here: for example, falling in love with someone wwith whom we feel deeply sexually compatible, but with whom we are deeply emotionally or intellectually incompatible. Again, because love can change us profoundly, it may be that such incompatibilities ccan be worked out between the people in love. But strong initial incompatibilities are very serious negative indicators, to be sure, especially because lovers are notoriously wishful thinkers--the experience of strong attraction always somehow seduces us into thinking that what isn't really right will somehow become right if we are just patient and supportive. Well, sometimes that works, but...
I like Nicholas's response as-is, but will chime in here with a book recommendation: Robert Solomon's book About Love is an absolutely fantastic work. It is written to be accessed by anyone interested in love, marriage, or relationships. It is wonderfully clear and has been the intellectual highlight of my summer. I think it would help you thinking through your decisions ahead. Good luck.
The texts of intimate relationships are generally too complicated to make judgments about using simple moral principles. But as a weakly stated general rule, I'd say that it's not wrong to marry or simply remain in a marriage out of a sense of duty. In fact, I would say that a sense of duty is a desirable element of a good foundation for marriage. It is, however, wrong to marry or remain married for the sake of duty but do so deceptively--that is, it is wrong to marry or stay married only or principally for sake of duty when your partner in marriage believes otherwise.
I can't imagine why you would think this is wrong. We're talking about consenting adults, right? If the relationship turns out to be very important to both of you, I would expect you would find ways to get back together again. Long distance relationships can be difficult, but they're not impossible!