I think that your preference is by birth, hormones, nature, however you want to say it. Whether you act on the preference is by choice.
Very few people are 100% hetero- or homosexual - most people have attractions or urges at one time or another, the only difference is how strong it is. That's why there are men who come out as gay late in life after being married & having kids. They might have been hetero *enough* to do that, but not for all time. And some gay people could probably never get through a hetero sex act.
On the nature/nurture debate, there was a famous sexual reassignment case that was reported on in Rolling Stone.
http://www.infocirc.org/rollston.htm
Hybrid Mode
