Having sex people people of the same sex is a choice for everyone who does that. Actually wanting to is the preference bit. There's no "for some" about it. I choose to get off with women because, well, I like it. I could equally choose to attend gay orgies. If, what you're saying, that some people choose different actions to their preferences, then you might be right, but that's usually the case when people's preferences are repressed by society. Ie, in the past people who were gay having to act straight.
He's not talking about choice as in who you choose to have sex with, but choice as in your sexuality. They are different things.