I will add on a final note that the fact that we are having this discussion here is largely thanks to the fact that the parade exists in the first place, and that that is an undeniable accomplishment of the whole movement and may by itself justify its whole existence.
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.
That's what the mainstream media tells you. There is this myth propelled by humanists that everyone who is a homosexual to any degree is genetically predetermined to be so, whereas many straight people lack these genes. Biologists understand that this is nonsense.
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.
All serious research and even common sense point otherwise. Same-sex relations were for example very widespread in ancient Greece. In modern Greece much less than 5% of the population participates in same-sex relations. Genetically, modern and ancient Greeks are similar enough so that if homosexual acts were a product solely of their genes they'd still be at it today. The reason why they don't is because social factors dictate it doesn't happen.
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.
If I got into the biochemical specifics of the issue I'd spend all day writing an essay here, but basically an increase in an individual's tendency to engage in same-sex relations can be attributed to either genes (unlikely for most people), different patterns of development caused by maternal hormones (in utero environmental factors), or environmental factors after birth.
Desperation and inability to have sex with women, for example, can lead to an increase in homosexual acts in many primates (ourselves included). This has been observed in chimps, and you have to look no further than jails and ships to see it happen with us. In South American military schools it was fairly normal for more masculine kids to use less masculine kids for pleasure. Culturally, the only 'homosexual' in that equation was the receiver.
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.
Only humanists would argue that normality is subjective. It isn't. But you must define it before entering a debate. If you are talking about statistical normality then exclusive homosexuality is definitely not normal - not in wild animal species, not in humans, exclusive homosexuals are very hard to come by. If you equate normal with natural, i.e., not a product of human mischief but rather a genuine expression of someone's instinctive emotions, then most human behaviour is normal. When I use 'normal' I use it in the statistical sense.
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.
But quite clearly as I stated in my previous post, I do believe that if bisexual behaviour occurs frequently when social norms don't constrict it (and it does) there must be an adaptive reason for that too. There are studies that show that with each male son, the next male son's chances of being gay increase. This shows that the causation of homosexuality in certain males may not be genetic, but rather environmental (if in-utero). Homosexual males may stick around closer to their families and act as agents of kin-selection. For the same reason, a genetic component increasing the strength of homosexual impulses may make its way into families fairly regularly (though based on current evidence and contrary to popular belief there is less support among the scientific community for purely genetic causes than any other alternative). Females who have homosexual brothers appear to be more fertile than females with heterosexual brothers, in which case male homosexuality may just be a collateral effect of a process through which mothers enhance their fitness by making fertile daughters after they've had sons.
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.
This doesn't mean that their likes, just like mine, weren't developed at least partly after they were born (not accounting for imprinting and gender identity is ludicrous). I'm not arguing that the extent of that equation doesn't vary significantly from one individual to another - for it quite clearly does.
Yes, sexuality is complicated and there are probably biological and social/cultural determinants at play for all, gay and straight and otherwise.
I once had a lesbian girl tell me that her need to dress like a man, get a man's haircut, etc. was genetic. I insisted it wasn't and got a 'homophobe' reputation among all her gay and bisexual friends. I've had similar arguments countless other times with LGBTQ people and it led me to conclude that mainstream LGBTQ organistations don't encourage critical thinking and empirical evidence among their members. Instead they encourage this 'everyone is against us, don't let them tell you that you weren't born exactly how you turned out to be'. Hence my lack of sympathy for the parade.
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.
