Happily bi, but also monogamous. My mate is the same. (I think there is a big cultural misunderstanding that being open to both means you necessarily need both at the same time, and so you can't be monogamous.)
As I look at sexuality, I don't accept 'straight' to be the default 'healthy program' in a person. Also, I don't accept that gays are simply born gay.
(I think some attention has to be given to de-constructing that argument, as it's become a gay battle-cry of sorts. I think gays were actually forced to take on the genetic predisposition argument for political expediency, to deflect nasty Stonewall-era rhetoric about them being willful sinners. Sure, it deflected the rhetoric, but it has turned a debate about rights into one of evading culpability for what the Right describes as evil actions. So we have allowed value to be taken away from our choices of who to love and how to love them, corrupting gay sexuality into some sort of imperative, derived from a genetic defect that forces us to sin. My genes are not defective, and my love is a choice, not a sin.)
No, I think we are born with the capacity to love others regardless of gender, and to be attracted by any number of things in our environment, and that we make conscious decisions on how to act on those drives; we are not simply puppets. There is a large amount of socialization that directs us to express these drives toward members of the "right" species, age group, social class, ethnicity, and apparent sex... rather than shoes or diapers or cartoon characters. Obviously, interesting things can happen in this socialization that yields variations. Although this identity is still a bit flexible down the road, it becomes pretty firmly entrenched. So I can see how a person might consider their sexuality genetic, due to its apparent permanence.
I'm pretty sure that if humans were raised without any socialization, they would be much more indiscriminate about their partners. Dogs can form a similarly deep affection for furniture and guests' legs until they meet with a rolled-up newspaper or spray bottle. I'm sure wolves learn appropriate targets from their pack as they grow up. Really, if people would speak more in terms of the choices they are making, instead of what their genes (or jeans) are telling them to do... it'd be better all around.