Addressing your question of free-choice, I do not think that we can accurately describe exactly what choice is. We can determine that sexuality is malleable, not from an isolated incident like the one you described, but the comparison of large groups (general society ratio of homosexuality versus amount of ex-prisoners with an acquired homosexuality).
See sibling post and response on the nature of objective, measurable qualities versus mutable qualities. At least, for some definitions of choice (something we do not have an accurate picture of), homosexuality and other sexually deviant behaviors meet the criteria for "choice." We could attempt to define choice further, but I think that is as far as that argument could go.
If it can be shown that homosexuality is a form of perversion or mental disturbance, then it would behoove you to be concerned with it just as we are concerned with other "sex culture," even if it is just cast as a general social opprobrium so as to diminish the contributing factors, similar to how we consider a prostitute to be of low morals (even if it doesn't "hurt" anyone). Personally, I do not consider prostitution to be a great boon to society and would not want to see a loved one enter in to it. This makes me concerned with the sex life of other consenting adults. Do you think your argument of "if it doesn't concern me and people are willing, it is none of my business" holds up to this kind of scrutiny? There are other laws that we have that would fall into this category as well such as drug use. To take it to a further end of the spectrum, consider drunk driving. Does it directly affect you? Are you concerned with it? If you think it directly affects you, what about driving while tired, elderly, distracted? These are only related to you in ways that are sometimes difficult to see directly, but they have a large effect on society when taken as a whole.