Traditional evolutionary theory suggests that being gay offers no advantages.
If that's true, it must surely be strange that same-sex attraction has been by no means unusual in humans since time immemorial and that it has been reported more recently in at least 1500 species of animals.
What's going on?
Scientists aren't sure but tonight on Channel 4 at 9pm, some of them discuss their ideas.
There's also input from a sheep breeder who tells us some rams are exclusively homosexual and it's very bad for business!
Well worth a watch, I think.