Besides AIDS and STDs, John Wayne Bobbitt and Basic Instinct have proven that. Every guy would love to have a loaded gun underneath the pillow which he can grab in case the woman has a sudden urge to lunge for an ice pick. After watching When Harry Met Sally, one has to wonder whether men and women can just be friends without ever thinking of having sex with each other. I think it's impossible. I have plenty of female friends who always get mad at me for showing the slightest interest in another woman who is a figure of modern pop culture.
Sexism is another interesting topic.
If a bunch of guys are sitting around talking and see a beautiful woman, they may comment on how beautiful she is. This could be termed as sexism. As if women don't treat guys as objects. Sure they do. If you don't believe me, try putting on the scene from Legends of the Fall where Brad Pitt is naked.