I'm liking Bob's argument... but I'm gonna play Devil's advocate for a second to give another perspective and say, since the second you are born you are exposed to Male-Female relationships, your parents, your aunts and uncles, your friends' parents, your cousins, etc etc, everyone around you is living a "straight" lifestyle, so you just follow suit and do like everyone you see, you chase girls, date girls, try to kiss girls, etc etc.... So if we use that argument we still haven't answered that question what causes homosexuality? Did gay people experience something else during their childhood?
Whatever, I personally believe you're born with it, just like straight people are born straight, just wanted to give another point of view!