Spend a little time with single women in their early to mid-30s, and you'll be grateful you're not one of them. The relationship scene is even more dismal today than when I was their age. All the women want serious relationships that lead to marriage, but many of the men they meet do not. All too often a woman moves in with some guy, hoping they're on the road to somewhere. Two years later, he tells her he's not ready for marriage and kids just yet. Splat.

But wait. Hasn't online dating made the mating market easier? Yes – for men. If you really want to hear a woman rant, just utter the word Tinder.

Single women are more equal and empowered than ever before. They have unparalleled sexual, reproductive and economic autonomy. In many ways, they're doing much better than the men. (Just look at the lopsided university graduation rates, which are now around 60-40). And yet, large numbers of young women admit their private lives are a sad mess.

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If you're a gender studies major, stop reading here. You're going to hate what I've got to say next. I don't like it much myself.

In a nutshell, over the past few decades, the traditional relationship exchange has broken down. It used to be that men and women each had something the other really needed. Men needed access to sex. Women needed access to resources. Men couldn't get steady access to sex unless they had resources to offer, so they worked hard for them. The partnership between men and women was a grand bargain that (usually) left both sides better off.

For men, sex was traditionally expensive. The price tag was a long-term commitment to provide for a woman (and children). But today, sex is cheap. And that changes everything.

This is the premise of a bracing new book, Cheap Sex, by American sociologist Mark Regnerus. Sex got cheap because of three technological developments: the advent of the Pill, which divorced fertility from sex; the onset of mass-produced, high-quality pornography; and the arrival of online dating sites, which make it easy for men to find willing sex partners.

Sexual liberation is a fabulous thing – in some ways. But it can also turn men into louts, because women don't expect much in return for access. Today, most men can have all the sex they want for very little cost – no fancy dinner required. The irony, as Mr. Regnerus writes, is that today's mating market is probably more dominated by men's interests than ever before.

When women complain that marriageable men (sober, steady good providers) are harder to find than ever, they may well be right. The marriage rate is falling steadily, especially among the lower middle class, while long-term stable marriage is increasingly a privilege reserved for the better off.

A lot of women seem to have their act together these days. But a lot of men don't. "I think the greatest, most astonishing fact that I am aware of in social science right now is that women have been able to hear the labour market screaming out 'You need more education'… and men have not," MIT economics professor Michael Greenstone says in Cheap Sex.

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What might explain this puzzling fact? Men don't have to prove themselves as providers any more. They can get all the sex they want anyway – including online porn on demand that can make the real thing feel mildly disappointing. (Ask younger women about men and porn. You'll get an earful.)

Like it or not, women have always been the gatekeepers for sex – not because they don't like sex, too, but because (no matter what you learned in gender studies) men's sex drive is innately higher. This means it's up to us to make the rules. "Why buy the cow when you can get the milk for free?" my father used to say. It drove me crazy when he said that. Now, it's dawned on me that he was right.

Since the women's cartel collapsed, women's bargaining power has seriously eroded. That's why so many single women hate Tinder, which has further commodified sex for the benefit of men. Women are just another consumer good in the shop window.

It may take a village to raise a child. But it takes a village to raise a husband, too. And modern society has largely abdicated from the job. "Good husband material doesn't occur naturally, but is instead the product (in part) of socialization, development, and social control," Mr. Regnerus writes. "[I]n the domain of sex and relationships men will act as nobly as women collectively demand."

Time to get our act together, ladies. If we don't, they won't either.