>>369065731
You are born gay, at least if you're male. Genes don't need to be involved, the clue is the fraternal birth order effect.

More older brothers from the same mother -> more likely to be gay, even if the younger son has never met his brothers. The main idea why is something-something progressive immune response in the pregnant mother. 

Might even be preventable once it's better understood, but imagine the controversy if parents started screening for homosexuality.