Simon: The reality is it's probably neither and both.
You are probably right. I'm no expert on sexuality, but experience has taught me that there is almost never a single cause behind any phenomenon; most effects are the result of multiple chains of causality that come together to form 'the perfect storm'.
I had one gay friend who confided in me that he was 'abused' as a child by several older males. I put 'abused' in quotation marks because he said that he really didn't consider himself a victim and in fact enjoyed every second of it. This man was not into boys at all; he liked big, hairy manly men! In his case I suspect that he was probably just born gay; though I'm sure a fundy Christian would point to his early sexual experiences as the root of his homosexual feelings. I think if he had been straight, he would have looked back on those experiences in very different light. The lucky bloke hit the jackpot!
But on the nurture end, I could use my own kink as an example: I'm straight, but I have been a hardcore masochist for as long as I can remember. When I was six years old I was already the sub of a very domineering and sexually precocious little girl who would molest me at our daycare when no one else was around. When I was seven I had a crush on my summer school teacher; a young woman who was as cruel as she was beautiful... I remember having sexual fantasies about her before I even knew what sex was. Could my early sexual experiences have predisposed me towards lusting after cruel and domineering women? There is no 'could have' about it.