I think the increasing number of gay people is due to social factor (more socially accepted, not condemned to death anymore, well, at least, in the Occidental part of the world) more than to genetics. Well, they may be related to genetics, but not in a "natural birth control way", because the "overpopulation" issue appeared so fast, I don't think evolution could follow and adapt so fast. I may be wrong, though...
There's always been gay people, they just didn't show off like we do today. Plus, homosexuality has been documented in over 450 vertebrate species... and their homosexual behavior is, there too, more socially than genetically significant.
Read the following interesting article : http://seedmagazine.com/content/article/the_gay_animal_kingdom/
written by Joan Roughgarden, an American evolutionnary biologist, and also, transgender.
I took out the most interesting quotes for all your lazy asses ;) ... "This means that same-sex sexuality—long disparaged as a quirk of human culture—is a normal, and probably necessary, fact of life"
"By neglecting all those gay animals, Darwin misunderstood the basic nature of heterosexuality.
"For too long, biology has neglected evidence that mating isn’t only about multiplying."
"My discipline (biology) teaches that homosexuality is some sort of anomaly. But if the purpose of sexual contact is just reproduction, as Darwin believed, then why do all these gay people exist? A lot of biologists assume that they are somehow defective, that some developmental error or environmental influence has misdirected their sexual orientation. If so, gay and lesbian people are a mistake that should have been corrected a long time ago. But this hasn’t happened. That’s when I had my epiphany. When scientific theory says something’s wrong with so many people, perhaps the theory is wrong, not the people."
"When animals exhibit homosexual behavior, they are just using their genitals for a socially significant purpose."
"You’d think by now, several hundred million years after sex began, nature would have done away with such inefficiencies, and males and females would only act to maximize rates of sexual reproduction. But the opposite has happened. Instead of copulation becoming more functional and straightforward, it has only gotten weirder as species have evolved—more sodomy and other frivolous pleasures that are useless for propagating the species."
"Being gay clearly makes individuals less likely to pass on their genes, a major biological faux pas. From the perspective of evolution, homosexual behavior has always been a genetic dead end, something that has to be explained away. (...) However, if one looks at homosexuality from the perspective of a community, one can begin to see why nature might foster a variety of sexual interactions. According to Roughgarden, gayness is a necessary side effect of getting along. (...) Roughgarden even argues that homosexuality is a defining feature of advanced animal communities, which require communal bonds in order to function.
"Of course, most humans don’t see sex as a way of maintaining the social contract. (...) Furthermore, it’s strange for most people to think of themselves as naturally bisexual. Being gay or straight seems to be an intrinsic and implacable part of our identity. (...) But if you look at vertebrates, that just isn’t the case. You will almost never find animals or primates that are exclusively gay. Other human cultures show the same thing.” Since Roughgarden believes that the hetero/homo distinction is a purely cultural creation, and not a fact of biology, she thinks it is only a matter of time before we return to the standard primate model. “I’m convinced that in 50 years, the gay-straight dichotomy will dissolve. I think it just takes too much social energy to preserve. All this campy, flamboyant behavior: It’s just such hard work.”... And a personnal favorite : "Having homosexual sex is the biological equivalent of apple pie: Everybody likes it."