Thanks to cheap sex, marriage may be doomed.

The share of Americans ages 25-34 who are married dropped 13 percentage points from 2000 to 2014. A new book by sociologist Mark Regnerus blames this declining rate on how easy it is for men to get off.

Regnerus calls it “cheap sex,” an economic term meant to describe sex that has very little cost in terms of time or emotional investment, giving it little value.

Regnerus bases his ideas, in part, on the work of British social theorist Anthony Giddens, who argued that the pill isolated sex from marriage and children. Add online pornography and dating sites to the mix and you don’t even need relationships.

The result is “two overlapping (but distinctive) markets, one for sex and one for marriage, with a rather large territory in between comprised of significant relationships of varying commitment and duration,” Regnerus writes in “Cheap Sex: The Transformation of Men, Marriage, and Monogamy” (Oxford University Press).

In generations past, women generally made men wait until marriage to have sex. To get a wife (and, therefore, sex), men had to be clean and presentable and have a good job. This, Regnerus reasons, gave men all the motivation they needed to become respectable members of society.

Now with porn on demand and greater reproductive freedom, sex is a commodity available at any time. This has left men with little motivation for marriage, writes Regnerus, who cites demographer Steven Ruggles’ prediction that one of every three people in their 20s will never marry.

‘Sex has become free and easy. This is today’s version of the opiate of the (male) masses.’ - psychologists Roy Baumeister and Kathleen Vohs

Regnerus blames cheap sex for the decreasing education and employment rates among men as greater numbers of women get college degrees and enter the labor force. Six percent more women than men in the 25-34 age group have a bachelor’s degree.

Regnerus backs this theory up with a quote from social psychologists Roy Baumeister and Kathleen Vohs, who study this phenomenon. “Nowadays young men can skip the wearying detour of getting education and career prospects to qualify for sex,” they write. “Sex has become free and easy. This is today’s version of the opiate of the (male) masses.”

Regnerus argues that while women have maintained their role as sexual gatekeepers, men control the marriage market. And given the ease with which sex can be accessed, Regnerus believes that men’s motivations for marriage have all but disappeared. He surveyed 15,000 people and found that among unmarried respondents under 40, “for every 82 men who wished to be married, 100 women said the same.”

This ratio, he says, keeps ultimate relationship power in the hands of men. “To plenty of women, it appears that men have a fear of commitment. But men, on average, are not afraid of commitment,” Regnerus writes.

“The story is that men are in the driver’s seat in the marriage market and are optimally positioned to navigate it in a way that privileges their (sexual) interests and preferences. It need not even be conscious behavior on their part.”

In turn, he writes, this leads women to settle, entering into doomed or otherwise unsatisfying marriages.

Regnerus even points to “Fifty Shades of Grey” to prove his point. In the book, Christian Grey gets Anastasia Steele to agree to a series of submissive conditions, including “any sexual activity deemed fit and pleasurable” by him, with no such power returned on her end. “I recognize that ‘Fifty Shades’ is fiction,” Regnerus writes. “It’s made up. But when you sell 100 million copies in two years, your narrative is resonating. There’s something to it.”

Meanwhile, many will go it alone. Self-love for men and women is at an all-time high. A 1992 study found that 29 percent of men (and 9 percent of women) masturbated at least once a week. In 2014, 49 percent of men (and 32 percent of women) confessed to doing it at least once in the previous six days. Unsurprisingly, “as frequency of [watching] porn increased, so did masturbation.”

All of this, Regnerus concludes, means that as long as sex is so low-cost for men, heterosexual women will have increasing difficulties finding a partner worth committing to.

“In the domain of sex and relationships, men will act as nobly as women collectively demand,” he writes. “This is an aggravating statement for women to read, no doubt. They do not want to be responsible for ‘raising’ men. But it is realistic.”