I served as a member of the armed forces of the United States of America under Don't Ask Don't Tell.
I chose, of my own free will, to not engage in the sexual activities to which I was predisposed, for a period of six years to preserve my standing and career until other circumstances rendered it untenable and I left. Before that, I maintained a strongly hetero-normative life to avoid the hostility that I felt would come from stepping out of the closet.
People choose to act on their sexuality, including heterosexuality, all the time. Often, they do not think of themselves as acting on anything. But you can tell that, for instance, heterosexuals act on their sexual orientation on a fairly regular basis on account of we're all here having this conversation.
Is sexuality a choice?
I believe it is. Or, rather, how we compose ourselves based on our sexuality is. We are all predisposed towards, to one degree or another, a given choice of sexuality. Being predisposed to a particular answer doesn't mean you don't have a choice, it just means you know the answer you /want/, or the answer you need to reach emotional and psychological consonance.
It may not always be the best choice under the circumstances in which you make it. It may not always be the safest. But it's your choice to make, and I don't think it's right to take that from someone any more than I think it's right for us to create a condition in which such a choice becomes so perilously difficult to make.
It shouldn't be any harder for a homosexual to choose to live openly as a homosexual than it is for a heterosexual to choose to live as a heterosexual...a choice so easy we don't even notice that it is being made every day.
But I don't believe it's right to deny someone agency, particularly over so powerful and personal a subject, by waving genetics and hormones and unilaterally declaring that they are helpless before the engines of their body. It's no righter to say that, than it is to say they make that choice to spite you, or that it's a trivial choice or to threaten suffering and damnation for making that choice.
That sort of coercion, to satiate any form of greed, is evil.
Why is it considered offensive to regard sexuality as a choice?
I believe Piorn has the right of it here.
Because most people who suggest that it is do so under the misapprehension that there is only one right choice, and that anyone choosing otherwise is doing it to spite them and (worse) to spite 'god', irrespective of whether or not the subject of the choice being made even factors such.
We should try not to be guilty of the same sin.