I'll admit I'm very much on the fence about whether or not being gay can be a choice, though I do believe that both are just as baseless as each other, with neither side being backed up by any SIGNIFICANT amount of verifiable, reliable scientific evidence. (One or two studies does not the well constructed argument make.)
I've observed a close friend's sexuality for quite some time, initially the most out there, open, psychologically and physically happy and healthy lesbian I have ever known in my life, slept exclusively with women, had no interest in guys whatsoever and wouldn't have even thought of sleeping with a guy.
Then, after about six years of secure lesbianism, she comes across this guy and is now exclusively with him and has been for the best part of another two years, hasn't even thought about going back to women.
From a technical standpoint, you could argue that she was never really "sure" of her sexuality (though from six years of sexual, mental and physical security I'd say that's a stretch at best), but the way I see it is that she found someone that meant something to her and made an informed choice to alter part of her lifestyle.
Though...Technically a case study, I can't go ahead and apply the findings from one person to every gay/lesbian that has ever been since ever.
I don't believe everything is quite so black and white, but then I believe that there are exceptions to every rule, though I suppose if pressed I'd probably side with sexuality not being a choice by and large.