Just because a couple plays house by cohabitating or taking vacations with each other's families, that doesn't necessarily equate to marriage, Seligson said. Her book, released earlier this year, combines scientific research and interviews with more than 160 couples who are "a little bit married." CNN asked Seligson about these lengthy relationships, cohabitation and advice for couples who are in long-term relationships.

Seligson: My baseline is one year in a monogamous relationship.

Do you see this as someone you are making big sacrifices and life decisions around?

This generation is radically rethinking straight sex and marriage, but at what cost?

In Part One of a two-part series, Rolling Stone goes under the covers in search of new approaches to intimacy, commitment and hooking up.

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Jehovah’s Witnesses view dating, not as recreation, but as a form of courtship, a serious step toward marriage.

So many people say to me in the clinic, that they don't need testing because they always use condoms...? But the gamification of modern dating is not the future, and it's not healthy. I work in the clinics and do these tests day in and day out, for my patients. In fact, it's all very well to give advice to other people, but when it happens to YOU, it suddenly feels very different.

“I like everyone to meet each other and be friends and stuff,” he explains.