Now here's a thing. Last week, scientists took what they say is the "gay gene" and injected it into some boy fruit flies (aptly enough). And do you know what those boy fruits flies did? They ignored the girl fruit flies and arranged themselves into flying conga lines - "Everybody change sexual orientation and party!" - before enjoying what one tabloid report breathlessly, and enviously, described as "an orgy".

Of course, orgies are in the eye of the beholder - maybe it was time for a slow dance, and who cares as long as they're wearing condoms - but I must say I found the in-flight Busby Berkeley stuff convincing. It sounds like something a gay fruit fly would do, and, Ripley's Believe it or Not, there actually was an experiment in the Seventies where piped disco music turned mice into dancing queens, so the movement and music link is obviously a given. But, unfortunately, the lab boys failed the rigorous standards of modern science and there exists no record of the fruit flies quoting Wilde and Firbank or discussing the relative merits of Callas and Caballe, or even Crisco and KY.

If that had been the case, then I could say in all confidence, you in the white coat, stop right there: the gay gene exists. You've proven it. I don't know why you wanted to prove it, but, hey, everyone should have a hobby. Alas, they didn't, so now we'll never know.

Not that I give a monkey's. (Monkeys, by the way, display gay sexual behaviour, no jab required. So do dolphins and whales. Then they stop. And start again throughout the life cycle. Or not. Gay gene or bloody- minded trendy bisexuals?) Everyone seems fixated on why homosexuals are homosexuals except this homosexual. Life and Tom Cruise are too short, know what I mean? Besides, has anyone tried making their way through the competing theories?

You twisted my arm. Freud says nurture not nature, blaming the absent father and the dominant mother, hence the immortal graffiti: My mother made me a homosexual/ If I gave her the wool would she make me one too? And let's not forget "pseudo-homosexuality", which isn't an intellectual pursuit, but when the bully father is the culprit. Still, where discovering that your only begotten son prefers his Pal to his Lassie is concerned, it's usually Mame who gets the blame, the gay child invariably thought to be "feminine" in some damaged fashion (don't spread this around the nation's leather bars). Poor old Mater also has her face on the Wanted posters for allegedly "passing on" the gay gene.

Naturally, every Bonnie needs her Clyde - that's the straight gene for you - and Night and Day magazine last year pointed an accusing finger at both parents, citing ... oh, this deserves to be quoted: "Night and Day has calculated that boys from divorced marriages can be four times as likely to grow up gay as those whose parents stay together ... There are assumptions built into some of our calculations [no!] which are supported by insufficent data to prove them beyond all possible doubt ... but, even without them, a trend exists ..." Two trends actually. The first is using figures supplied by the University of Toy Town, the second is tracing the root cause of homosexuality back to - brace yourself, it's a big one - heterosexuality. Hey, thanks! We couldn't be doing it without you ...

Too pop psychological? OK. Brainy Simon LeVay says that one of the four clusters of neurones in part of the anterior hypothalamus is, on average, three times larger in heterosexual men than women and that the cluster in gay men is the same size as that in women, a blow for size queens everywhere. Whereas Ulrichs and Hirschfield posited that homosexuality stemmed from the female soul being trapped in a male body - which makes about as much sense as my dad claiming it was because I didn't play enough football (apparently all that time spent in the showers afterwards doesn't count).

What else? The Nazis believed it was lack of testosterone, while endocrinologists used to say it was lack of testosterone during pregnancy. This might appeal to those who believe chemistry explains everything except Tammy Wynette's dress sense. But frankly it's not half as much fun as the Catholic Church's logic - corruption - or a one-night stand with the fundamentalists' fall guy: Beelzebub himself, wearing horns, moustache and lumberjack shirt, for surely Satan himself is responsible for the clone look?

So many theories, so little time ... and we haven't even touched on Kinsey and the behavourists, who opt for habit and opportunity, which would at least explain why male prisoners adopt homosexuality with an ease verging on the eager - oops, dropped the soap again. And it introduces the concept of choice, that sexuality might not be monolithic, one, static state, but who's awake at nights over this? Is anyone sleepless over what causes heterosexuality? Hardly.

So what can I tell you? I can tell you I'm gay because the money's great and the hours are your own and that, from personal experience, the one thing I have ever known to cause homosexuality is alcohol. Honest. I've lost count of the married/ engaged/nominally straight blokes I've woken up beside only to hear, "Boy, was I drunk last night!"

File it under proof: 5 per cent.