See there is a claim that requires a lot of proof.
one I could find on quick notice Though I could also point to the likes of Anne Heche who went from Hetro to Gay to Hetro again.
Do you think sexuality is a choice?
If so, what did you choose, and why?
No and yes. I think sexuality is a very complex thing, and a small part of it is choice, though the vast majority of it is a function of our genes and how our environment manipulates our genes turning them off or on via our life experiences (hence the choice part). This would result in the possibility of having "gay genes" in your DNA, but not actually having them activated because of the environment and experience of the person. It could then lead to later experiences activating them, or deactivating them in a person with them previously active.
As SezMe says, this makes it very hard to change, but not impossible, and it has been shown that some people can and do change their sexuality throughout their lives. The trouble is that we like to label people, if you are attracted to the same sex, you are gay, if you then decide that you like the other sex too, or find an attraction to them, you're bi, etc, however is reality it's a case of our sexuality fluctuating as we grow and we respond to our enviroment.
This means that if you make the choice to immerse yourself in a different sexuality, then over time that environment may write a change the genes that are on and off in your DNA, modifying the way your body responds to that environment. The question is, does everyone have the same genes, or do some have only "gay" and some "straight" genes meaning that they are harder to manipulate? That I can't answer, though I actually suspect we all have one set and how they are switched on or off determine what our sexuality is at that point in out lives rather than some people having "gay" ones.