I don't know of any feminist writer who would assent to the claim that "all men are sexist." I seriously encourage you to think about where you got the idea that it is common for feminists to think such a thing. Feminists have had always had to contend with people caricaturing or willfully misunderstanding what they say, and so there are a lot of misconceptions floating around. If you are seriously interested in feminist views, I would suggest that you start reading. I'd suggest, as a start, the book *Discovering Reality* by Marilyn Frye.
I cannot speak for all feminists, but I do hold views that are pretty common among feminists, so let me tell you my reactions to the claim you mention. First of all, I consider sexism to be a structural, rather than an individual problem. It is not primarily a problem about the false beliefs or malign attitudes of individual men, and much more a matter of an entire system that gives women a much more limited menu of life options than men have. This is evident in the fact that, in every society, at every time in history, and every place on the globe, by any measure of life quality, women are worse off than men. It's an open question -- but one for empirical investigation, not a priori philosophizing -- how this system got set up in the first place, but it's likely it has to do with conflict over control of women's reproductive capacities. In any case, the existence of these systematic limitations on women means that men, in all societies, at all times, etc. enjoy unearned advantages over women. In that respect -- and listen carefully to this -- men all have an interest in the preservation of sexist structures, *even if* they bear no *personal* responsibility for the structures' existence.
Imagine it this way -- suppose you have purchased, in good faith, an art object that turns out to have been stolen. If the theft is discovered, you will have to relinquish the art object to its rightful owner, *even though* it may not be possible for you to get your money back from the thief. Thus, you would be better off if the theft is never discovered. This is true *even if* you are the kind of person who would, upon discovering the theft, immediately return the object to its rightful owner despite the cost. Many men are like this with respect to sexism. They say: I renounce the advantages that I have just in virtue of being a man rather than a woman, and I commit myself to dismantling the system that gives me such advantages. But the first step to renouncing those privileges in recognizing that they are there. That's hard.
The second thing I want to say about the claim you're concerned about is that it's possible for a person to contribute to the maintenance of sexist structures without intending to do so, or without realizing that he (or she) is doing so. There is a great deal of work being done now in psychology on what is called "implicit bias" -- psychological attitudes that, without our being consciously aware of them, influence the way we interact with other people, the inferences we draw, and the evaluations we make. There is a great deal of evidence, for example, that men and women evaluate the same job application differently depending on whether the name at the top is male or female. I do not like to characterize these findings by saying that people who do this "are sexist," because people are not responsible for their unconscious attitudes in the same way they are responsible for their conscious attitudes. Now that we know about unconscious attitudes, however, I do think that everyone has an obligation to work to uncover and change whatever unconscious attitudes one might have.
Everything I've said, by the way, has a parallel with white people and racism. I am a white person, and so I have enjoyed unearned advantages over black persons. The rage many of us feel about recent killings of unarmed black people is connected with this -- I am not the least bit worried when I am approached by a police officer on a dark night; I don't fear that he or she is going to view me as dangerous, and shoot me pre-emptively. (This is one rare situation in which my being a woman makes me safer!) White people who "get it" about the systemic and unconscious nature of racism respond by working against racism, even though it is (in the sense I explained above) not in their interests to do so.