I agree our community does more harm than good by insisting that sexuality is utterly immutable. The fact is that we live in a highly diverse world. The transition of a bisexual from a same-sex relationship to a heterosexual relationship is often viewed as proof of change. The recovery of a sexually disoriented youth to their natural orientation is viewed as further proof. Sometimes, transgendered people change orientation when they begin hormone treatment. Why is it so difficult, then, to imagine that hormone levels in others cause changes in their own orientation. Not too long ago, there was a case of a man who claimed that head trauma turned him gay. If sexual orientation is determined by brain structure, his claim should not be so surprising.Regardless, I think reasonable people can disagree over the mutability of sexual orientation. The part I disagree with is the notion that people who are gay *should* want to change, and are in need of therapy to right a wrong.
That orientation is fixed is fundamentally true. Most of the observable counterexamples have nuanced explanations. It's not worth walking people through those explanations when they're really not interested anyway. We may as well just acknowledge them and move on.
It is better to merely say that no one controls whom they fall in love with. And when you do fall in love, you have the right to get hitched for better or worse.