I think people make their own choices on the sexuality. There are bisexuals, homesexuals and heterosexuals. More people are embracing their sexuality, realizing it is nothing to be ashamed of. A few years back, it was shameful not to love someone of the opposite sex. More people now are accepting that love is love.
It certainly can vary as a person grows and learns. People are constantly changing and what they are attracted to can change, as well. This is not saying that it happens for everyone, but for some people a fluid sexuality is normal. In general, changing sexuality is less common in the world population, but it does happen.
I think even someone who identifies as homosexual or heterosexual may, over time, have attractions to someone of the gender they aren't primarily interested in. I think this is probably because we are attracted to someone as a human, not a sexual object who happens to walk around and talk.
No because scientists have proven that our DNA is a roadmap and can show every detail of how someone is, including sexuality. So unless there is a line of DNA that says our sexuality will change, our sexuality will always stay the same. For hard evidence go to the link below: