Ok, I am bi. I have known so for 6 years. It is not a choice.
But that is not to say that I was born bisexual. There are two reasonable arguments to this debate. One, which many have already stated, is that you are born straight/gay/bi. In others words, Nature. The other one, and the one that receives my support, is that it is rooted in your early relationships and experiences of the world. The Nurture argument. When I say early I mean 0-7 years old.
Gay males tend to have been far closer to their mothers in this period compared to their straight counter-parts. My ex was constantly abused by males and received more support from females. She is a lesbian.
~ The journey of a thousand miles begins with a single step ~
* Nothing in the world has ever been accomplished without passion *