I was just reading about psychotic diseases in identical twins, and if one has a disease, for example schizophrenia, then it's only a 50% chance that the other twin would get it. I think all diseases, sexuality, habits, obesity tendencies, etc are genetically predisposed but triggered by the environment. So if one twin is gay, it's not a 100% probability that so is the other.

I'm not always right, but I'm never wrong :P - Garfield