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Nobody has made the argument that men aren't allowed to have moral beliefs. The difficulty is that, just like with women, it's difficult to tell when a man has a universal moral belief or when he has a belief that only applies to the opposite gender. If someone has a problem with threesomes - male or female - he or she is entirely entitled to that belief - but it had damn well better go both ways or it is sexist. The men and women that feminists have a problem with (and yes, we find many women as sexist as men) are the ones that think it's ok for guys to sleep with whoever they want but think that girls should remain "pure." No reasoning, biological or otherwise, justifies that belief - you're entitled to have it, but don't be surprised when women argue with you about it since we are on the receiving end of your ignorance.
For the record, I do not have a problem with threesomes (though I've never had one), but my boyfriend thinks this is vile. We disagree on this point, but I do not run around calling him sexist for having it because, as a rational man, he applies this belief to everyone, including himself. I had slept with more people than he had when we started dating and though he doesn't want to know the details, he's fine with it. On the other hand, I once dated a guy who put me into the "whore" category because I had slept with three people before him even though he had slept with 15 before me - needless to say, that relationship didn't last long because he WAS sexist. Those are the type of guy that all of us crazy feminists have a problem with.