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the gene (or geneplex) is very old - older than the gene for blue eyes, say.
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Don't assume that something has to be selected for in order to exist. We can do calculus, even though that obviously was not selected for. Nor would there necessarily be strong negative selective pressure, since critters can both reproduce and have gay sex. Possibly, or rather, likely, some important trait that was selected for has a side effect of sometimes causing same sex attraction.
Birth order effects odds of being gay, suggesting it's not genes per se, but the womb environment, and therefore still of biological origin.
There is not a clear explanation for WHY homosexuality would emerge from genes, but there are strong indications that it does.
If I remember right, identical twins raised apart have a tendency towards having the same sexual orientation.
Oral histories of gays are very telling. Time after time in coming out stories people describe their desperate attempts to be straight, often lasting for decades. For them, it's clearly of biological origin.
Wiki on Penguins: "Male penguin couples have been documented to mate for life, build nests together, and to use a stone as a surrogate egg in nesting and brooding. In 2004, the Central Park Zoo in the United States replaced one male couple's stone with a fertile egg, which the couple then raised as their own offspring."
The Wiki entry has some interesting ideas:
http://en.wikipedia.org/wiki/Homosexuality