I always seem to see this from bisexuals. Believe it or not, there are people out there who are strictly or very nearly strictly attracted to one or the other (I'm one of them). For these people, labels can be helpful. They can forge a community. Anyway, if you put off identifying yourself as gay, in my experience others will do it for you. Then what are you gonna say? "We're all bisexual." That won't be very helpful. To answer the OP, there is certainly no "obvious" evolutionary disadvantage. The studies coming in, for example from Italy last year, indicate that women who have gay kids also have more total kids. For these families, gay does serve a purpose, at least if you remove what is really unnatural - homophobia. This is also means that we are probably not a "check" on population control after all. Straight parents are predominantly the ones passing down the epigenetics/womb environment that leads to homosexuality, and it's been that way all thru human history. In addition, i believe it's been determined that kids born of homosexuals are no more likely to end up gay. It's not simple mendellian genetics at work, but it is biological.
There are several ideas that try to explain homosexuality. I don't think it can ever be fully explained. It might be the inner ear as explained by medical science. It might be the devil inside us as explained by religion (chill out, that's a joke). Personally I think it is a natural form of birth and gene control. I also think there is influence from hormones and chemicals in our food, water and environment.
Everyone, indeed all animals "are gay." It is just a matter of how latent the drives to engage in the behaviors are.
From what I have read all humans are girls in the womb until the fetus gets hit with testosterone. This is why men have nipples. It is a left over trait of being a girl at the beginning of the pregnancy. The testosterone comes in two phases. The first time it changes girls into boys physically and the second time it changes the female brain into a male brain. So a lack of testosterone in either phase results in a person who might have one type of body and another type of mind.
I would agree with the above. Even if sex is determined by chromosomes, a combination of genetics and environmental factors would change estrogen and testosterone, which have been linked to sexual orientation. For instance, in the aforementioned exclusively homosexual sheep, heterosexual rams have a certain level of chemical that develops into estrogen into testosterone. On the other hand, homosexual rams have the same level of this chemical as in female sheep.
