In the 21st century, we have decoded human DNA, we can examine the stars of distant galaxies and we have even begun to unlock the myriad mechanisms inside the human brain, but of one universally natural phenomenon we still know virtually nothing. That phenomenon is homosexuality.

When I was a teenager trying to find out how or why I turned out gay, there were very few reference books to help me. The phenomenon itself had been scientifically documented by the American social scientist Alfred Kinsey — but he proffered no explanation of its origins. It had befuddled Freud.

The scientific consensus had been laid out in the Wolfenden report in 1957, which solemnly concluded that homosexuality was “compatible with full mental health”. But the tantalising questions endured. Are people born gay? Is it genetic? Is it related to hormonal variations in the womb during pregnancy? Could it be affected by early childhood environment? Or is it a function of some other unknown factor?

We still don’t know. All we know is that it appears to be fixed by about the age of three. This lack of precision is partly due to the complexity of the phenomenon. Who knows whether sexual orientation isn’t multi-determined by any number of genetic, environmental or hormonal factors?

But our ignorance is also due to ideology. Neither the right nor the left has really wanted to know. Anti-gay social conservatives have long been uninterested in research that might prove the genetic basis of homosexuality — because it would “normalise” it. And the post-modern left insists that sexual orientation is socially constructed, and so scientific research is irrelevant. This politically correct right-left pincer has essentially slowed research on homosexuality to a crawl. Even now, a gay teen has barely more knowledge than I did 25 years ago.

Yes, there are some recently uncovered, tantalising genetic clues, gleaned more from the maternal DNA lineage than the paternal one. There are studies pointing to clear differences between homosexual and heterosexual hypothalamuses in the brain.

There’s a huge new volume of data about animal homosexuality, revealing it to be ubiquitous and complex. And as more and more gay people have emerged into the sunlight of public life, the range of their own stories has added to our collective understanding of what being gay, in all its varieties, can mean.

The trouble is: whenever science gets closer to figuring out the puzzle, politics intervenes. And so last week, Martina Navratilova and the usual suspects protested against new research on gay sheep being conducted at Oregon State University.

The researchers have been adjusting various hormones in the brains of gay rams to try to see if they can get them to be interested in the opposite sex. The indifference of many rams to otherwise attractive and fertile ewes is a drag on sheep-breeding, it seems. We don’t have any peer-reviewed studies yet, but reports of success in manipulating the sexual behaviour of some rams have led to an outcry.

The gay rams have a right to be what they are, Navratilova complains. She may be a little defensive about the breeding of farm animals — but you can see her broader worry. If you can figure out how to flip the gay switch off in sheep, how long will it be before someone tries to do the same in humans?

The good news, then, is that the empirical origins of sexual orientation are slowly being discovered. The bad news is that once discovered, they could be manipulated. There seems no likelihood in the foreseeable future of a hormonal treatment that could affect sexual orientation in adult humans. It’s been tried to no avail for decades — and once drove great men like the brilliant codebreaker Alan Turing to suicide.

But it’s not unimaginable to see scientific insight into the origins of animal homosexuality being abused if directed towards human beings in their first months and years. Maybe hormonal manipulation in utero could make homosexuality less likely in a sheep — or a child. Or maybe in the future, research like that being done now on sheep could be used to detect homosexual orientation in foetuses or babies — and prevent it. Why not, if that’s what parents wish?

The answer, of course, is an ethical no-brainer. Experimenting on other human beings crosses a bright moral line — even when that other human being is in your own womb. There is no medical reason for meddling with anyone’s sexual orientation, let alone in the crucial first months of a human being’s life. And the potential for all sorts of unintended consequences is huge. Most ethical doctors would abhor such practices. And rightly so. Laws could even be passed, and enforced, to ban them.