See, that's what I think part of the problem is. We've built the whole idea on the notion that no one can choose. It's a defensive stance. It reinforces the idea that only those things that are beyond people's control should be protected instead of using a better measure of things-that-don't-hurt-anyone should be protected. In general that's not a good way to look at the world. "It wasn't my choice" is not very far from "It wasn't my fault."
I also think it has the potential to alienate anyone who is confused or questioning or curious about their sexuality. Right now the message is: when you're young it's okay to be unsure, but once you grow up you'll now what you are and that'll never change. If someone does have some change in their feelings or identity beyond those initial years of puberty the message we put out there says "Well, if you think you're gay now it must mean that you were gay all along and didn't know it." I don't think that's a fair thing to say to a person. I think it ridicules their life up to that point by implying they were using a "false" sexuality or something along those lines. I'd also hate to see someone who might feel curious about their sexuality being forced back into a "No, I'm straight, I can't" mentality. That's bad for them mentally and emotionally.
I get that it's easier to get people to allow something if we say "It's not a choice," but that marginalizes people within an already marginalized group.