Here’s my take on this issue. I think that in our societies that have been (and still are to an extent) very repressive towards homosexuals and have been totally lacking in presenting an homosexual relationship in a positive way, and on top of that have considered anything except strict monogamy as not socially acceptable, the number of men (or women, whatever) who have been involved in homosexual relationships has been (and still is) artificially reduced to the bare minimum of people who are completely entrenched in homosexuality, and have no interest altogether in heterosexual relationships. That would be most of these 5% (or whatever other figure someone else thinks is more accurate)
However, even amongst us, remove women (for instance in jails) and suddenly the homosexuality rate increase a lot, and if despite it being still perceived negatively, showing us than a non-insignificant number of supposed heterosexuals aren’t completely adverse to homosexuality, despite the stigma.
Now, let’s climb in the time machine and go back to Athens. Let’s assume further, despite these points being disputed in this thread, that there’s no stigma attached to homosexuality. Since you’re born, you never heard of having an homosexual relationship being a bad thing. Not only that, but the perception of free women is such that loving a man might be more “manly” than loving a woman.
Now, like in our modern jails, remove all women from the scene (apart from slaves, that you might pick as a sex toy, but aren’t going to consider worthy of your affection). They’re nowhere to be found in the public sphere. You aren’t going to interact with them socially. They won’t ever be present when you hang out with or visit your friends. You aren’t going to meet any during your leisure time. Finally, you being married isn’t in any way preventing you from being involved with men at the same time (while having an affair with a free woman would be a very big no no). I’m pretty certain that in such circumstances, the percentage of men being involved in homosexual relationships would skyrocket.
Now, allow me to go anecdotal. As a teen, I was mostly undecided regarding my orientation, with however a preference for girls. The more time passed, the more I erred towards heterosexuality. I would say I’m… maybe 97.5% heterosexual. Me being involved with a man at any point in the future, is, if not completely unthinkable, at lest very unlikely. I never even had sex with a man. I definitely am not in your 5%. But if I had lived in a society such as the one I described above, I suspect I might have had much more homosexual relationships than heterosexual ones.
Actually, if homosexuality was or had been much more accepted and widespread, I might be now a full blown bisexual, even in our current society where there are plenty of women available. The two first serious shot I had at having a relationship, or at least some form of sex, have been with boys. Both time, it was prevented by well-meaning third parties (one of these interventions still piss me off to this day. I really liked him. And, boy, was he hot! But I digress).
If they hadn’t been prevented, since I never had dated a girl at this point, I would have begun my sexual (and possibly emotional) life with boys. And that might have been sufficient to have me switch to the other side. I don’t know. Since I’ve been progressively less and less attracted to men, maybe I would have been progressively less and less attracted to women if I had not established intimate connections with them later, if I’ve been hanging out mostly with homosexual men, etc… I would be, maybe, in your 5%.
To sum up, I think that though 5% (or 10%, or whatever) might be a valid figure for active homosexuals + closeted homosexuals with zero interest in women, I believe that the percentage of men with a potential for homosexuality is much higher, and would be even higher in a society with a very different perception of homosexuality and offering much more opportunities for homosexual relationships.