From what I saw so far it looks like it is not a choice. Some people speculate about ressesive gay gene that shows up in some people, others have other theories. The fact is people just notice one day that they are atracted more to same sex than to opposite, rather than make a concious descidion to become gay just for the hell of it.
There are gay animals too, if you havent noticed, so like it or not, but gay are aboslutley natural. You can say that their activities do not lead to making kids, but even with estimated 5+% of gay people Earth is about to get over populated, so I dont think that this is such a big factor after all.