Actually, as I think about it, I believe the tendency is something that you are born with, in the sense that is manifests itself in some and doesn't in others. However, this does not mean that someone who was born with it cannot control it. As a Christian, I've heard of Christians who have gay tendencies and are working to fight them -- we don't consider it to be a 'sin' unless they get into a homosexual relationship.
At first I was convinced it was a choice, but Mace MacLeod's post made me think. It's true that I couldn't feel love for another male even if I "tried" to. So it is obviously something that some feel and others don't.