Sexuality runs deep in the human personality and those men who are straight can't relate to getting erotic with another man. The thought disgusts them and therefore they think it's a perversion and therefore immoral. They don't have the sophistication or the compassion to try to put themselves in the shoes of a gay and realize that, for gays, an erotic connection between two males isn't a perversion but feels natural and is an expression of actual love, or to realize that gays don't have a choice on the matter, it's just the way they're wired and therefore they shouldn't be condemned for wanting to have a love life.
And it pretty much about men hating on men who get with other men, notice that men never really disparage lesbian porn, even homophobes probably love it, and the harshest condemnation of gays seems to come from males rather than from females.
Though some females are anti-gay too (and probably less intensely), probably mainly because their Christian indoctrination tells them that gaydom is a sin. It's a shame that bigotry against gays is hard-coded into the Bible, thus just reinforcing people's inherent homophobia by elevating it to the level of an absolute righteousness. Why is it hard-coded into the Bible? Probably for the same reason men tend to be homophobic today. The Bible is just a projection of our own moral values thousands of years ago onto our notion of God.