Sexual Orientation as Counterfactual

I sat in on our Gender and Identity class the other day and have been pondering this question: is sexual orientation a behaviorist notion (based on what you've done) or is it counterfactual (based on what you would do)?

Determining what it means for a counterfactual to be true is a philosophical conundrum all its own, but in this case the behaviorist approach seems to have some problems of its own. There is a poem in Allen Ginsberg's later writings in which he describes a sexual encounter with a male student who was clearly trophy hunting -- the young man was clearly excited to be able to say that he had sex with Allen Ginsberg, but was noticeably not excited while actually having sex with Allen Ginsberg. The student was straight. If it wasn't for the counter-cultural credentials, he would never have wanted to engage in this activity. But he did, even though he is very unlikely to do so, or even to desire to do so, again. Does this encounter make him bisexual?

If sexual orientation a matter of what you've done, we run into cases like this. If it is what you would do, how can we determine anything since it seems not possible to know what one would do given that you haven't done it. What would count as evidence? Is it what you've had a desire to do? In this case, our desires would have to be transparent to us. Are they? Does self-reporting become perfect? Or is it a counterfactual here, what you would have a desire to do? I have no idea how we would make determinations like this. How do we determine sexual orientation? It seems an important question to be able to answer if we want it to afford legal protection (although most cases would revolve around perpetrator's belief about the sexual orientation of the victim -- right or wrong, one could easily see cases where the question becomes relevant).