I think you have a mix of genetics, sub conscious, conscious, along past life experience that lead different people to different sexual identity choices. In some cases those choices were voluntary choices in other cases they were non voluntary choices driven by both genetics and in some cases on a sub conscious level.
Yes a straight person can choose to be gay just as a genetically gay person could choose to be straight.
Note the above does not imply that I think all gay people should just try and force themselves to be straight because honestly once you have a sexual identity that suits you you should just keep it.