One could make any number of cases as to why anyone would choose that. Attention grab, trying to be different, stick it to God, whatever.
What really makes the idea that you choose your sexuality a silly one is that it implies that one has control over what turns them on. Anyone who has ever had a boner should see the problem there.
But even then, to accept that you can't doesn't mean it's over, because the other side can argue that the choice comes into play when deciding whether to submit to or abstain from it. Except they don't. Homophobes don't care if gay people engage in absolutely no homosexual activity, they think it's something that can be changed, that homosexual can be conditioned to become heterosexual if they just try.
That's nuts