You've never met me, shadowfax 
But seriously, this is a bit of a circular conversation, because there's so many layers of nature, nurture, experience, conventions and trends that go to make up a person's sexuality. I don't believe any of us can truly claim we know the essence of our sexuality - because that essence doesn't exist. So researchers and theoreticians can go on doing their stuff, but the only sensible thing for the rest of us to do is to accept that everybody's sexual orientation is what they say it is, accept that sometimes people change, and get on and do our stuff.
When I first came out, over 30 years ago, people were always challenging me on it: it's just a phase, but you've had boyfriends, you don't look like a lesbian, you're too feminine to be a lesbian etc. (I think people do this a LOT less now.) So I got kind of deliberately uninterested in any conversation about why people get to be gay, or whether we are all really bisexual etc. We are what we choose to identify as - that'll do for me.