I think that there is actually a sense in which both sides in this debate are right:
For some people, homosexuality is not a choice. They have strong, exclusively-homosexual attractions to their own sex as soon as the sex hormones cut in. The powerful and exclusive nature of their attaction, coupled with our inability, after a century or so of study, to find an obvious environmental cause, strongly suggests that their sexual orientation is has biological roots. (Maybe genetic, maybe something else.)
But for almost all other people, homosexuality is, in a sense, a choice.
Men, or women, put together in groups and deprived for a long time of any possibility of contact with the opposite sex, will turn to each other.
Not all will, perhaps. And thus here we can speak of "choice".
Note that this is not "true" homosexuality, but rather homosexual behavior. Homosexual behavior was not uncommon in the ancient world, but "true", exclusive homosexuality was not even recognized as a phenomenon. The concept of someone "being" a homosexual stems from the 19th Century.
Men in this situation will instinctively seek out those other men who look, and/or act, the closest to women. Even where female-deprivation is not an issue, some men may find themselves tempted by young just-post-pubescent boys, who in many respects have female characteristics: hairlessness, innocence. This explains the occasional Boy Scout leader of otherwise impeccable heterosexual provenance who gets caught "interfering with" his charges. He probably has a dessicated domestic sexual life, and is also inhibited from seeking the usual consolations of men in that position.
So: for some people homosexuality is not a choice, and for some, it is.
If you think it isn't, then you are in the category of "possibly will behave in a homosexual manner given the right situation".