One study found that up to 70 percent of married men and women have cheated on their partners.It’s one thing to tacitly accept that till-death-do-us-part monogamy may not be a realistic goal; it’s another to eschew exclusivity altogether.

When Tiffany met her longest-term partner, Phillip, in 2010, she told him right away that she wanted an open relationship. With just a few exceptions, they’ve been happily polyamorous ever since.

• AS THE STORY goes — at least, the one we’ve been told by the likes of Leave It to Beaver — Americans were once happily monogamous, both protected and isolated by the social contract of the nuclear family.

She once broke up with a girlfriend of three years and started what would become an almost year-long relationship with someone else in the span of several hours.

Being polyamorous, she says, without a hint of irony, has helped her figure out how to get her needs met without losing herself in the process.

And now, thanks in part to Tiffany’s organizing efforts, polyamory is having something of a moment among Philly’s under-40 set.

Tiffany and Phillip first came up with the idea for Polydelphia about a year and a half ago, after attending a series of unsatisfying meet-ups.

But since the beginning of human history, there have always been people like Tiffany.

In the late 1960s and early ’70s, multi-partner relationships thrived on the streets of Haight-Ashbury and in suburban living rooms alike, unbeknownst to most of the world.

The numbers prove there’s growing interest: As many as 12 million Americans practice some form of consensual non-monogamy today.