This is up for debate. How do people become straight? We'd like to think we know, but the areas in the brain corresponding to sexual impulse and attraction are incredibly poorly understood. There's actually (ironically) more scientifically supported evidence for an inherent cause for transgenderism than there is for homosexuality.
Either way, it shouldn't affect the way you treat 'mos. If someone participates in an activity or lifestyle you find disgusting, or believe is unnatural, but does not affect you in any way save having to know about it, what license do you have to inhibit their ability to do so? That's an abuse of power and an unnecessary exercise in stopping 8% of the population from participating in an activity or lifestyle on the grounds of opinion alone, and that's not right.
TL;DR nature v. nurture is a moot set of points.
