Magical Realist
Valued Senior Member
There are a number of theories as to how homosexuality (or traits related to it) were adaptive in early human populations. I'm not expert on it, but all one has to do is Google "evolutionary biology of homosexuality" and you can find several hypotheses quickly.
For example: http://www.tcs.cam.ac.uk/uncategorized/homosexual-sapiens/
Not that anyone can say for certain which, if any, is right.
The most common theory that I have seen is a version of the one above: that strong same-sex bonding is useful in a social species because it enhances group cohesiveness, and that homosexuality is the result when those traits become over expressed. That is to say, they liken it to malaria. Having one "gay gene" is good because it enhances the social bonds around you, but having two copies means you will prefer same-sex relations exclusively.
So why wouldn't being "half gay" be favored over being "fully gay"? Seems having two copies of one gene would quickly be selected out in favor of only having one copy of the gene. And have scientists traced gayness to double genes anywhere? Seems it would be something easily detectable.