Men are assumed to be more interested in sex than women, they are socialized to be the sexual aggressor, and they are rewarded for being sexually active. We define intercourse as penetration by a penis, and we believe the sex act is complete once the male has orgasmed and ejaculated. Most pornography is aimed at a male audience, although this is changing somewhat. And in general, men are rewarded for being masculine and for being heterosexual. They are encouraged to find women sexually attractive and visually stimulating, and women are encouraged in turn to attract and stimulate men.
Men are the aggressors, but they are also the enticers - both the chasers and the chased.
Gay men find other men sexually attractive and visually stimulating, instead of women, and I think there's an irrational fear held by many heterosexual men that all gay men are out there hitting up straight guys for sex. And since being manly is rewarded, being less manly is penalized. I also think there's a whole myth built up around anal sex . However, straight men are socialized to focus almost exclusively on their penis as an erogenous zone, and are not taught to enjoy other areas of their body like their nipples, their thighs, their anus, etc. There is also a theory that those who have the strongest homophobic reactions are themselves worried about their own homosexual feelings.