It's very easy for women to turn down sex. We turn down sex on the way to the subway. We turn down sex buying light bulbs at the grocery store. Just now, while typing this sentence, I turned down sex. Men have a trickier time of this. See, men are supposed to want sex all the time. It's supposed to be tattooed on their brains, as some kind of involuntary function, like breathing or not asking for directions. And if they do try to turn down sex, women too often treat it like a bizarre aberration -- like there's something wrong with them or (more chronically) wrong with us. Of course, sometimes men just aren't in the mood. Sometimes men are tired. Sometimes men drink too much Jameson and pass out on the couch.

Nevertheless, it seems that men may be turning down sex in larger numbers. According to a story in the Telegraph, one sex therapy clinic in the U.K. has seen a 40 percent uptick in the number of men who just aren't that into sex with their partner, especially men in middle age.

"Men used to come to us with impotence -- now known as erectile insufficiency -- but Viagra has sorted some of that problem. What we have is a lot of men who say, as women did in the 1950s: 'I can have sex but I do not want to. It's not rewarding.'"

Yeesh, I don't know why this is happening. Maybe it's depression, or "changing sexual roles," as the story so vaguely suggests. Maybe Viagra ripped the veil off a deeper sexual malaise. But it's clear the old notion about men wanting it all the time just isn't true. Turns out, two can play the "Not tonight, I've got a headache" game.