Sexuality isn't strictly black and white, there's lots of shades of grey. I think most people if they thought about it
could say that they've seen a member of the opposite sex and of the same sex they find attractive in some way
Social factors, however, push us to define ourselves by one basic label. You could be a woman that feels a strong emotional and physical attachment to women, but still sometimes find a man attractive. You could be a man that insists that he's straight and has only ever been straight but develops a deep emotional intimacy with another man.
So while I don't think true orientation can be changed, I think people's inherent bisexual nature can cause us to choose one label or another; I think a lot of bisexual men out in the world a rampantly homophobic because they think gay people can /choose/ to be with an opposite sex partner, since they feel attracted to both but call themselves straight. So someone "turning" gay is really just someone shifting that label to something more comfortable, not inherently changing.