Apart fom having the "specifications" from birth it's also a "talent" you can develop or repress. So I think both actually, with more emphasis on the "gene". You're born with it and then it's a matter of choice as well? ( but it's not really a "gene" is it? anyway, natural cause )
But if nature meant this to happen as something normal, then gays would be able to procreate and then all would be well. But it isn't.
now that's a lame, smart-assed answer, innit? 