Sex, love, and the rules that govern the two are fleeting, amorphous constructs that shift from person to person. That said, they can provide a window, however one-dimensional, into the "average" couple's lives.
When it comes to relationship longevity as it relates to when couples first have sex, Toni Coleman, a Washington, D.C.-based psychotherapist, speculates that three months after meeting a partner, or whenever the honeymoon phase is over, is the best time for couples to become intimate. This is simply based on the idea that they're more likely to have established the emotional chemistry required to form a true and lasting bond, rather than focusing solely on the physical.
"You move past [the honeymoon phase] and your feet are more on the ground," she told Business Insider. "...Sandra Metts' study said the couples who waited until that level fared a lot better than people who had sex on the first, second, or third date."
The study she's referring to analyzed the relationships of around 300 Illinois State University students in the early 2000s. It ultimately found that couples who said "I love you" before having sex outlasted the couples who did the reverse. Taking the traditional relationship steps, i.e. getting to know each other, sharing a first kiss, building commitment, and then having sex, was linked to greater relationship longevity.
Of course, this is just one professional's opinion based on a small, highly specific study—matters of the heart are hardly one-size-fits-all. What's more, these "traditional relationship steps" are becoming less conventional with new generations. Case in point: Millennials are 48% more likely than past generations to use a one-night stand to establish a romantic connection.