I'm sympathetic to most of what Professor Heck says, if we consider things from a deontological or even a consequentialist point of view, where the relevant consequences are external to the agent. Fantasy does not violate anyone's rights, and fantasy that never motivates action will not result in actions that harm anyone. But I think there is a plausible way of looking at things that would still find fault with fantasizing about having sex with children, and that would come from the aretaic (or virtue-theoretic) way of thinking, according to which the primary bearer of value is to be found in characteristics of agents. One who indulges in fantasies about sex with children is doing something that both reflects--and also perhaps perpetuates and sustains--a certain trait of character that we might think is not entirely wholesome or admirable. To the extent that we can regard one who indulges in such fantasies as having a trait of character that is improvable, we might also think that some attempt to eliminate or at least diminish the inclination to indulge in such fantasies would result in that person having some improvement in character. It may be that habituation can only go so far, and that virtue theorists (such as Aristotle) overrate the extent to which one can habituate better character traits, but it certainly does seem that a virtue theorist could find the character of someone who tends to indulge in such fantasies at least improvable, and this way of looking at things does, I think, put a different face on this kind of case than what Professor Heck has indicated.
The main thing wrong with the argument is that it is terrible. Don't we think it's wrong for parents to have sex with their children precisely because we think that it is harmful to the children? One might also think that children have no genuine capacity to consent to sex, an issue that also arises in other settings, such as between a boss and an employee. In such a setting, there are always issues about coercion, even if such coercion is not explicit.
Presumably the thought is supposed to be that there are forms of sex that are morally suspect, even though they do not cause any sort of harm. But then one wants to know what those are supposed to be. Then we could consider whether and why they are morally suspect. The example given, as I said, is a very bad one.
The question asked here is interesting, but not in my area of expertise, I'm afraid. I would, however, like to say something about the background stated with the question.
The most familiar form of the doctrine to which you are referring is known as "substitutionary atonement". It was introduced in the twelfth century by Anselm and has since become central to many people's thinking about the meaning of Jesus's death on the cross. For all I know, it may be the official doctrine of some denominations that have official doctrines, such as Catholicism.
This doctrine does have some basis in scripture, but it has been and continues to be controversial. One reason is that many people find it offensive that God should require God's own son, Jesus, to suffer a violent, agonizing death, no matter what its alleged purpose. And the idea that God would require atonement for wrongdoing to take the form of physical suffering seems to many people to make God out to be some kind of cosmic child abuser. As the great American preacher Hosea Ballou put it:
"The belief that the great Jehovah was offended with his creatures to that degree, that nothing but the death of Christ, or the endless misery of mankind, could appease his anger, is an idea that has done more injury to the Christian religion than the writings of all its opposers, for many centuries. The error has been fatal to the life and spirit of the religion of Christ in our world; all those principles which are to be dreaded by men have been believed to exist in God; and professors [of the faith] have been moulded into the image of their Deity and become more cruel...." (Quoted in Proverbs of Ashes, p. 30)
These themes are developed in several excellent books, including Stephen Finlan's Options on Atonement in Christian Thought, Mark Heim's Saved From Sacrifice, and, most powerfully, Rita Nakashima Brock and Rebecca Ann Parker's Proverbs of Ashes.
I recommend this last book to everyone, Christian or otherwise, because Western culture has embraced so much of this theology. As Brock and Parker argue, our culture's unthinking, reflexive tendency to blame victims of violence for their own suffering is, to a large extent, rooted in this sort of theology. Which is to say that this theology arguably condones violence, especially intimate violence.
All of these are good questions, but we should distinguish two issues. The first issue is as to the moral facts. I claim that if you come to know that use of some term is offensive to the members of some group, then you ought not to use that term. It's an entirely different question, of course, whether someone's telling you does lead you to have such knowledge, or whether your refusing to believe them might be unreasonable, and so forth.
Questions about how many people in the group find use of the term offensive are another matter. We might well suppose, for example, that some people of gypsy descent are unaware of the origin of the term "gyp", or that some American Indians might be unaware of the origin of the term "Indian summer". Others might be aware of the origin, but think very few other people are, and so not themselves find use of the term to be offensive. So, well, it's complicated. I haven't thought about this deeply, but I'm inclined to think that the issue here isn't one of numbers, though, as it is of what is reasonable and unreasonable. I.e., even if it is only a few people who take offense, if they do so on perfectly understandable, reasonable grounds, then that is enough, whereas, even if many people take offense, but do so for silly reasons, then that seems different. And, of course, people might disagree about what's reasonable. But these things can be discussed, and my own view, practically speaking, would be that one should err on the side of the offended.
All of that said, I think the worry about "walking on eggshells" is unfounded. I did say, explicitly, that one would not normally be blameworthy for using a term in ways others (reasonably) found offensive. Of course, there is a kind of wilful ignorance that might make one blameworthy, but I don't think you have to take the initiative here, and make sure you're not using any term in a way that someone might find offensive.
I think the answer depends very much upon what one thinks one is doing philosophically. But the important point here is that moral claims in particular, and many of the philosophical claims that get evaluated using these invented examples, are meant to be more than just true as things actually are. So, for example, the claim that it is wrong to torture babies just for fun is meant to mean not just that all the actual baby torturing that is done just for fun is wrong, but that any baby torturing that is done just for fun would be wrong. Counterexamples to that claim, therefore, do not have to involve actual cases.
As the philosopher Timothy Williamson has pointed out, moreover, many of the sorts of hypothetical counterexamples philosophers use either do have real-world instances or else such instances can easily be created. There are, for example, some very famous examples concerning knowledge known as "Gettier cases", and, at the start of one of his papers, Williamson cleverly sets up a situation (without letting on that he is doing so) that is, in fact, a Gettier case. It's hard, as he points out, to see that his setting up an actual such example has any philosophical significance. And, sadly, I do not think the situation described here is either wildly speculative or extremely unlikely. Such situations have in fact occurred. I cannot remember where, but I vividly recall reading some years ago about a father who was forced at gunpoint to rape his own ten year old daughter in front of his entire family.
Nonetheless, the ordinary concepts we human beings deploy to understand and navigate our world developed under very specific historical and environmental conditions, and it would not exactly be a shock if those concepts had a tendency to "break down" if we imagine the historical and environmental conditions being very different from what they are. If so, then that might make one skeptical of really obscure and implausible sorts of examples, e.g., ones that depend upon the supposition that someone has magical powers, etc, etc.
If I'm reading the question correctly, it assumes that if morals aren't empirically verifiable, then they must be based upon emotion rather than reason. Frankly, I don't know why anyone would make that assumption. There are lots of important claims that aren't based upon emotion, but that ultimately aren't empirically verifiable. For example, the claim that 'if morals aren't empirically verifiable, then they must be based upon emotion rather than reason' is not empirically verifiable but does not seem based upon any emotion.
If any of the great modern philosophers had the 'last word' on ethics (and a vast range of other issues) it would have probably been Kant who wrote after Hume and rejected many of his views (including this one).
Following up on Heck: The church I attend (Episcopal) is quite welcoming to gays. The associate pastor (and for many years my confessor) is a Lesbian priest. There are substantial support groups for homosexual Christians in different denominations. While Richard Swinburne is a Christian philosopher who has serious reservations on the merits of homosexuality, his book Revelation provides a goof philosophical framework within which to take Dr. Heck's advice and see the meaning of the Bible / revelation as something that is on-going and progressive.
Just a brief comment, which is that, even if one always does have a moral right not to do things one regards as morally objectionable, it does not follow that one has a legal or political right to do so, i.e., that one cannot legitimately face legal consequences for not doing so. It is fairly easy to come up with counter-examples by thinking of people whose moral views are themselves pretty objectionable. I don't know which issue the questioner probably had in mind, but both seem worth considering.
For what it is worth, I think similar such examples make Aquinas's view very doubtful. If one thinks a thing is morally abhorrent, say, when it is, in fact, morally obligatory, then it is not at all clear that one has a moral right not to do the thing. Indeed, it seems almost contradictory to say one does: One has a moral right not to do something that is morally obligatory? Presumably, the resolution of this "paradox" lies in distinguishing subjective from objective elements of this, as is now pretty common.
I think science can probably tell us lots of things about how people reason morally, that is, how they think about what they ought to do. And it might well be interesting to look at cross-cultural differences, and perhaps even more interesting to look for cross-cultural similarities, that is, "moral universals", in the sense of moral principles, or forms of reasoning, that are in some sense universal. Psychologists and philosophers have been doing just this in recent years.
But it seems important to recognize the contrast you cite at the end of your question: No such investigation could possibly tell us what moral behavior ought to be, that is, tell us what one actually ought to do. Suppose there turn out to be certain "moral universals". It would be a coherent position that these are just wrong, that is, that, by reasoning in accord with them, one will not typically arrive at the thing one ought to do. One cannot just assume otherwise. That is not to say that it would not be interesting, even on this view, to know about these "moral universals". It would mean we would have to work especially hard to combat them!
That said, I'm bothered by the phrase "moral behavior", which is very different from "moral reasoning", which is what I've just been discussing. Consider this claim, which you cited as a provisional observation: "Almost all moral behaviors are strategies for increasing, on average, the synergistic benefits of cooperation and areunselfish at least in the short term." How is the scientist supposed to know which the moral behaviors are? I take it that a "moral behavior" here is supposed to be one that is morally appropriate: right, or good, or whatever. Whether a particular act is moral in that sense isn't the sort of thing one can in any plausible sense observe, nor even design an instrument to measure. Presumably, then, the scientist will have to judge which behaviors are "moral" using his or her own sense of morality. But maybe she's just wrong. Note that this problem is the one that is supposed to be avoided by founding scientific investigation on observations that are, at least to some significant extent, "theory neutral": scientists are at least supposed, at least ideally, to be able to agree on the data, even if they disagree about how the data are to be interpreted. The problem is that there seem to be no data for a scientific theory of "moral behavior".
So, science can (and, as I said, currently does) investigate what different societies believe about moral standards, and investigate how different people think about what one ought to do. But I doubt science can, even in principle, investigate what you are calling "moral behavior", that is, investigate moral standards themselves.