I think it is a choice. Heterosexuality is the norm - the blueprint, so to speak - for humanity. It’s how the species reproduces into the next generation. Since no one has yet produced evidence of a “gay gene”, I have to go with the logic that says that deviation from the norm is by choice.
What causes the choice? Damfino, and I’m not terribly sure the people involved do as well. It’s the sum of all the parts of their lives to date, and it’s different for everyone.