Has anyone mentioned epigenetics yet? It's really interesting (and complicated). People think the cause of being gay is involved in that. For example, identical twins are genetically identical, yet there have been documented cases of one being straight and the other gay. I'm not saying they made a decision, but just that there was some other factor...
I don't think It's genetic in the sense that a LGBT parent creates LGBT kids (obviously this isn't true for most of us). But I think hormones in the womb could play a factor.