IMO, people don't "decide" their sexual orientation. People decide whether or not to act on it, whether or not to "come out of the closet", so to speak, but I do not believe one's true sexual orientation is chosen. I'm not talking about casual experimentation, many if not most people have done that. I believe sexual orientation is decided most likely in the womb.

And to answer the poll, I'm heterosexual and have always known.