One of the most obvious reasons I think is that I just didn't wake up one day and consciously decide to be gay.
I was struck by a beautiful young woman and immediately knew I was in love and then wondered how in the hell that could happen to a "supposedly" straight, married, 27 year old, 8 month pregnant woman?
I know it is natural, because having sex with a man was always embarrassing to me, and never felt right, but oh my God, when she touched me, I knew what was right and everything was answered for me.
That is just me...I can't answer it from a scientific viewpoint.
And those who say it is a choice, I wonder if they chose to be hetero?