I think you’d better not laugh at the idea that homosexual preference may have evolutionary benefits. […] it’s well within the scientific mainstream, and worthy of study and understanding.
So is social darwinism, and the reason people don't take social darwinism seriously these days has little to do with whether the idea has intellectual merit or not. But I think this thread is not about biology so let's drop it.
intolerance of gays is contrary to the essential moral tenets of the major religions
I have been thinking about this discussion, and it seems that someone made the very valid point that religious people do not consistently follow the teachings of their religion. So they choose to harass homosexuals supposedly based on what the bible says, but completely ignore the bible when it comes to, say, their choice of clothing. That is, indeed, a very strange behaviour.
An answer to this is difficult to find, I can only relate it to my experience as a Catholic. To pick one example, Catholics are not supposed to use contraception, yet the majority of them do. To the best of my knowledge, the reason we use contraception is the same reason we don't respect speed limits: because it makes no sense. And when I think of every decision in my life, I find that I'm not guided by my religion at all, it's only that there is a lot of overlap between what I believe and what my religion teaches. If one day too much dissonance develops, you can be sure I will drop the religion rather than change my beliefs. Isn't that in fact what we see happening all the time?
So back to the issue of homosexuality, in light of what I said above the answer seems to be that a lot of people just don't tolerate homosexuality, religion having nothing to do with it. The overlap is purely coincidental.
This guidance leads me to wonder why so many religions consider homosexuality bad
I was a bit disappointed that no one paid attention to my comment regarding sex between female teachers and male students, which is not harmful to any party yet strongly forbidden nonetheless. The secular society is just as pruddish about sexual behaviour as any church, only they pick different battles. I can also think of sex with minors, poligamy, interbreeding, pornography, even the completely innocent exposure of female breasts in public can arouse violent opposition.
Let's face it, we are obsessed with sex. When we are not doing it, we are thinking about it. Just look at this thread. If we don't control our obsession it will turn our civilization in an endless orgy. The problem is, we haven't come to a consensus as to what and how to control.