"Genetic predispositions"? We're still at the "maybe, maybe not" stage on whether that even exists. It's especially dicey when the "definition" of gay is itself cultural. Many languages don't even have a word for it; in contrast, there are cultures which assume that a predisposition to sodomy, as to drunkenness or gluttony, is more or less universal, and goes by the name of "concupisence."
"Puritans, Mongols and Muslims"? Show me the survey of gay Puritans, correlated with the lab results for the sequence of the gay Puritan genetic marker. Published by a reputable Plymouth Bay Colony journal prior to 1690.
Puh-leeeez.
Even if you grant that it exists, the question that follows is, "So what?"
One could then argue that serial killers are genetically predisposed to violence. Does that excuse their actions? Do we, as a society, then grant them blanket immunity because they were "born that way?"
I reject genetic predisposition as an excuse. Whatever weaknesses you may or may not be born with, you are still accountable for your actions!
We are pretty convinced that alcoholism is a genetic condition, yet we still expect alcoholics to avoid drinking and driving or buying liquor for minors. So, the question of whether homosexuality is genetic or not is entirely moot! It's homosexual acts that are morally offensive and inherently wrong. This also dismisses the straw man "Don't tell me who I can love" argument. It's not about who you love, it's about who you have sex with!
Regardless of your genetic predisposition(s) or emotional leanings, you can still choose whether or not to commit homosexual acts. That's where the trouble lies.