Ross Douthat admits that sex outside marriage cannot be avoided. But he emphasizes that there are "different kinds of premarital sex":

There’s sex that’s actually pre-marital, in the sense that it involves monogamous couples on a path that might lead to matrimony one day. Then there’s sex that’s casual and promiscuous, or just premature and ill considered.

Dana Goldstein tweaks Douthat's message:

I agree with Douthat that many young people would be wise to abstain from sex until they are in a monogamous relationship with some lasting power. But I think from there, he goes seriously astray. I don't think it makes sense to teach teenagers that, ideally, they should be marrying the first (or second) person they ever sleep with.  What we should teach young people about marriage is that it's worth waiting for the right person: you know, a loving, committed, emotionally mature and stable partner with whom you share interests, goals, and all that other heart-warming stuff.

What I find unpersuasive is Ross's dichotomy. There's either pre-marital monogamy or everything else: "casual and promiscuous, or just premature and ill considered." In real human lives, isn't this a spectrum rather than a binary choice? Many men especially, to my mind, should be wary of marrying the first person they fall for or have sex with. Experience in relationships, and sex, and the tangled mashups of both, is surely salient for getting the marital choice right. This can end in tears, of course, but so can premature monogamy.

There is nothing lonelier than a bad marriage made for good reasons; and nothing but experience that can help you figure out if you are making a huge mistake.