I am starting my reply with your final comment because at the end of the day this is what the Pride debate comes down to for me too, among other things.
You make it sound as if this is some academically unchallenged and tired old chestnut that's been floating around since the beginning of time. Not so. 'Mainstream' science (which I take to mean established and peer-reviewed), medicine and psychology have reached this understanding over time and only recently... and not necessarily that there's a 'gay gene' we can point to, per se, but that there isn't a 'straight' gene for that matter either.
Social taboos may prevent people from acting on desires/drives but this doesn't mean those desires and drives don't exist, which is sort of the point.
The flaws in your point above are:
1) You cannot prove that the number of people participating in same-sex relations in modern Greece is 5%. At best you can only identify how many people openly acknowledge doing so. These are very different things, and these are slippery problems for science which 'demands' quantifiable numbers to be acceptable as valid. Instead you have a 'hypothesis' which many would refute.
2) The 'serious research' into sexuality in Ancient Greece that you refer to is also not quantifiable. Never mind the lack of statistics and so on, how the Greeks defined same-sex relations may cover everything from man-on-man hot times to something akin to the modern-day version of a 'bromance'. In other words these things may have been 'idealized' and celebrated but we don't really know to what extent they reflect true numbers.
To be honest you still do not make a compelling 'scientific' case that would refute the 'mainstream' position of science on this issue. On the contrary, you are hanging on to dated ideas about these things that have been discredited for some time.
Your points above really only underscore the complicated diversity of human sexuality and the inadequacy of our limited labels to try to understand it. You're absolutely right that a prison inmate who engages in homosexual acts isn't necessarily a homosexual (in the 'Kinsey 6' sense of this term) but is also not strictly 'heterosexual' either if we are using an inadequate terminology that forces sexuality into a binary opposition, in which case we would have to assume that under no circumstances would a heterosexual engage in same-sex acts, and no matter what the context... or else they aren't 'heterosexual'.
... and as for whether homosexuals are 'formed' in utero or afterwards we simply just do not know, but we don't know this about the formation of sexuality in general either (straight or otherwise). Reproduction is an act, and it is a same-sex act but just like your prison inmate there is many a gay dude who can carry out that reproductive act, and has! In other words reproduction isn't limited to heterosexuals.
We already know that homosexuality isn't 'normal' in the sense you mean it, but so what? Albinos and red heads aren't normal either but this doesn't mean for one instant that they weren't genetically/biologically predetermined to be albinos or red heads. In other words, 'normal' (as you define it) isn't germane to the argument in any way.
In fact there have been many studies that demonstrate quite the opposite of what you argue. There are established statistics that seem to point to the genetic determinants of sexuality in differing contexts (twins, number of male children etc). 'Nurture' in this instance has been largely discredited.
Yes, sexuality is complicated and there are probably biological and social/cultural determinants at play for all, gay and straight and otherwise.
I would just ask you to reverse the role. Imagine that you are a statistic 'minority' in a largely homosexist world, and then imagine that homosexuals were constantly trying to tell you how you should understand your sexuality, and constantly telling you that your heterosexuality is a 'construct', the mere result of environmental factors. No matter how you look at it, or how innocuously you intend it, there is something reductive and dismissive in this viewpoint. Given the far longer and wider context of history and homophobia a little more sensitivity may be needed on your part, is all.
I never said homosexuality was a construct. But it'd be naive to say that many heterosexuals
and homosexuals aren't letting constructs get in the way of reality. As a straight person who is incredibly attracted to women I can't convincingly say that if I'd been brought up in a social setting where certain homosexual acts were the norm I would be disgusted by them. On the contrary, from what I know I'd be inclined to bet I'd happily participate in this bisexual scenario. I'm honest about this with my friends, and you should see that puts ME in an awkward situation seeing as people tend to be horrible and judgmental - but I couldn't care less because I'm pursuing the truth and I'm very comfortable with how I carry myself out.
From what we know, homosexuals shouldn't torture themselves over their sexuality. It's a naturally occurring phenomenon, frequently in-utero, that shouldn't stop individuals from enjoying themselves sexually and socially. It also shouldn't stop them from assuming that many aspects of their sexuality may be plastic!
I am not making any 'arguments', I have limited myself to stating empirical verifiable facts. By 'mainstream' I obviously don't mean peer-reviewed articles, but rather stuff that makes it to the newspapers like The Star (which is my fav. newspaper anyway). Most peer-reviewed articles support that homosexuality doesn't appear to be caused by genetics judging by all the evidence we have available. If you are under the impression that scientific peer-reviewed articles support a genetic basis for homosexuality then you've been getting your information from the mainstream media as opposed to the journals themselves. Your accusation that my information is outdated is either downright defensive or ignorant. The greek example is valid in spite of your nit-picking, but if you so insist I can point you just as easily to the Etoro people, where homosexual acts are encouraged between youth and elders (young boys are encouraged to perform oral sex to elders to take in their 'force'). Homosexual behaviour in nearby tribes with genetically equivalent populations may be even more pronounced or completely non-existent - pointing to the lack of a genetic basis for the behaviours (they all do reproduce heterosexually however, pointing strongly to a genetic basis for that).
Twin studies prove that indeed there is a significant link between brothers who share a placenta to turn out gay. Studies also show that genetically identical twins who don't share a placenta show no such relationship. This supports that homosexuals can frequently be a product of in-utero environmental factors.
This is not a matter of what you want to believe, emotions are irrelevant. If an albino says 'my skin colour is normal' he is wrong. Just like if I were to personally state that my allergic reaction to alfalfa is normal would also be wrong. All of these things are of course natural and we shouldn't be treated differently for them. I must add there's a huge case to be made for bisexual behaviour being considered normal, though. There is an equally strong case to be made for bisexual behaviour to be considered 'genetic'.
When I was 15 I was diagnosed with a couple of hormonal disorders that have been running in my family for ages. I was told it wasn't my fault, that it was just genetics acting, and that all I had to do was to take a couple of pills every day for the rest of my life. I was 'normal' they said, to make me feel better. Luckily enough I incidentally wanted to study evolutionary biology, and before I knew it I got to understand what was causing these disorders with a precision that is not available to 99% of those who suffer them. I saw that apes in the wild very rarely suffered these same disorders, and determined that environmental and lifestyle factors must have been responsible for my body's malfunction. With this knowledge I drastically changed my diet and lifestyle in accordance to what evolutionary theory suggests should rid of these diseases, and in the span of a year I had stopped taking pills and my blood indicators were all optimal. Understanding without any prejudices the nature of what my body was going through allowed me to drastically improve my quality of life, and I encourage all sorts of people to use this approach to enhance their own life. If people were so convinced of the '100% genetic always for everyone' nonsense they wouldn't be so defensive about it.
Those who say 'I used to be with a woman but I'd kiss her and feel nothing, but I kissed a man and I felt something therefore I am clearly genetically gay' are missing the point completely and can't be taken any more seriously than straight people who say 'even if it was socially encouraged I would never let another man touch me with his dick ever'. I can't get any pleasure whatsoever from kissing a fat woman (in fact I'm positively sexually disgusted by them), but that doesn't mean I'm genetically predetermined to find them a terrible turn-off either. Sexuality is a complex subject but we surprisingly do know a lot about it. There are many methods you can use to learn a lot about yourself and experiment with what we think we like and don't. The conclusions you reach are irrelevant to me, and you may find that you are happiest having sex solely with leather-strapped indian males, but I do think approaching the issue with an open mind and critical thinking is key to building better and happier communities.
Currently in a place like Ontario a huge number of people let a list of convenient human constructs dictate their sex life. Christians and Muslims act like it's immoral to do anything outside of marriage with anyone other than a rather boring segment of the opposite sex. Asian/Indian immigrants harass their kids into not dating someone from an ethnicity other than their own (and more often than not succeed in drilling this in them). More liberal minded people settle for the myth that our sexual orientation is 100% genetically predetermined and eventually whatever we like can be justified with the magic 'genetics' card (I used to do this when I was younger, too).
For what it's worth, I'll preemptively state that I'm convinced most homosexuals didn't 'choose' to like members of the same sex. Please don't take my words out of context to imply I'm not.