There’s something going on with marriage in America. At a time when it’s available to so many, it appears to be taken seriously by so few.

Take Marnie Michaels, the willowy, statuesque musician played by Allison Williams in HBO’s hip series, “Girls.” As devoted viewers who watched Sunday’s episode are aware, Michaels has gone from single and dating to married, cheating and demanding a divorce in less than one season, treating marriage like it’s an exotic semester abroad.

Michaels isn’t the first “Girls” girl to try a marriage on for size. Back in Season 1, boozy, free-spirited Jessa Johansson met, married and divorced a rich — yet square — venture capitalist in less than a year. Three seasons later, Johansson is (finally) sober and “recycle dating” the ex of one of her best friends.

These women’s stories would probably be entertaining if they weren’t so downright depressing. For whether they call it a “starter” marriage, “beta” marriage or “test” marriage, the 25- to 35-year-old generation has a far more elastic definition of the concept of “forever.”

How elastic? A recent study found that 43 percent of millennials supported a form of marriage that allowed couples to easily split up after two years, while a full third were open to “marriage licenses” valid — like mortgages — for set periods of time. It’s an impressive figure, especially when you consider just a third of respondents still believe that marriage is “till death do us part.”

So what’s going on here? Have social media and dating apps killed off marriage? Or has digital culture — if not hook-up culture — so spoiled young people for choice that they’re simply unable to settle down? With same-sex marriage now legal, has making marriage more inclusive eroded its traditional sense of exclusivity? Or are millennials merely early-adopting a future where marriage is unnecessary?

Part of the problem is role models. Just 26 percent of millennials are married, according to a landmark Pew Center report, compared to 36 percent of Gen X-ers 20 years ago — and 48 percent of boomers back in 1980. Millennials are also among the least religious Americans ever — with a full third unaffiliated with any single faith.

In fact, a lack of faith is, perhaps, the most defining millennial characteristic: Just 19 percent of them believe that most people can be trusted. Unwilling (and probably unable) to rely on each other, millennials — like the “Girls” crew — are giving up marriage rather than giving it a chance.

And at what price? There are the economic costs — divorce, even with minimal assets, usually doesn’t come cheap. Then there’s the psychic toll of beta-testing marriage. For Michaels, this meant abandoning fidelity the first moment something — or someone — more appealing showed up. In her case, it was a former flame who’s devolved from a successful app developer to a grungy heroin addict — not that we’re judging!

What’s troubling about Michaels — and the generation she represents — isn’t that she broke her vows so easily (cheating is hardly age-specific). Rather, it’s the ease with which she broke her marriage — without fighting, without counseling and (most likely) without looking back. The real question is whether her decision will ultimately be without consequence.

Probably not.

Yet another recent study revealed American millennials to be among the best-educated — though least-skilled — demographic groups in the developed world. Compared to both their developed-world counterparts and older Americans, millennials suck at basics like reading, math and technology. The result? A millennial workforce worrisomely ill-equipped for the marketplaces awaiting them.

These same data points could also be applied to marriage. Raised in the shadows of their parents’ divorces and counseled (if not coddled) through every life conflict, millennials should have all the skills needed to negotiate their way to nuptial bliss. But much like their overpriced university degrees, the millennial approach to marriage is more theoretical than factual.

Or even practical. And so we have folks like Marnie Michaels who call it quits in Round One — a generation of men and women who never learn how to battle for their marriages and probably never will.

For most of American history, “starter” marriages — and their subsequent divorces — would stain and stigmatize literally until the grave. But today, they’re merely early-adulthood indiscretions (almost) as forgettable as a Facebook status update.

Like the social media that so rules their lives, millennials’ approach to marriage is all here and now — rather than here and forever. But while marriages may be easy to erase, millennials will learn the hard way that pain and loss are not so simple to swipe away.