Probably because it usually IS only feminists who purposely bring this sort of inflammatory topic up in coed company, and frankly, most feminists are very dishonest in the way they address these issues, usually exaggerating them to the point that even stuff like looking at a woman's cleavage that she purposely chose to expose is seen as a type of sexual harassment or objectification, saying good morning to a woman is seen as cat calling, and shaking her hand in a way that felt "creepy" to her is seen as sexual assault or molestation. Even spreading his legs when he sits down is construed as a type of oppression against women.
In other words, the reason a lot of men are getting pissed when they hear these things being discussed is because they are no longer about real victimization and sexism, they are about trying to demonize and accuse men of things they're not even doing.
How would you like it if it was common for men to purposely bring up conversations in mixed company that make it seem like most women are gold diggers and hoes who essentially oppress and mistreat men at every opportunity, yet use examples that are so exaggerated and taken out of context that it makes it clear that all they're really doing is purposely trying to find a reason to dislike and talk bad about women?
Your last sentence said that you reaaaaally want to know why they act like that, which is why I gave you a very honest reason for why they're getting angry. Now it's up to you to decide if the reason you asked this question was to truly try to understand their perspective on this...or just as a way to somehow further convince yourself that they're only doing it because most men are just jack@sses who don't like to be called on their bad behavior. The choice is up to you.