There are lots of reasons the US marriage rate has dropped to 31 percent, but a rarely mentioned one is the cost.

The figure comes from the National Center for Family and Marriage Research at Bowling Green University, which also says the average woman now first weds at 27.

That makes me thoroughly average, though my fiancée and I haven’t set a date yet.

But, while my Facebook feed is starting to fill with pictures of high-school acquaintances’ new spouses, most of our friends aren’t there yet.

And part of it is the outrageous numbers behind the big day.

The average wedding costs somewhere between $28,000 and $30,000, reports wedding-planning site The Knot. It’s more if you want to get married in the big city. (Lots more: I’ve looked.)

Of course, that’s far higher than just a generation ago. My own parents, who will be happily married for 30 years in September, held their reception in a small restaurant on Long Island. They were in their late-20s and knew they couldn’t afford a huge event.

But these days the wedding industry plays on the bride-to-be’s fantasies, increasing expectations to ridiculous heights, and has $40 billion a year to show for it. If you’re a bride bent on an extravaganza to put all others to shame, you may have to wait until you have more money saved — or take on a huge debt.

Yes, some parents can throw a huge wedding for their kids, but these days the entire middle class is stretched thin, whether you’re 25 or 55. Relying on the father of the bride has, increasingly, gone the way of the dowry.

There are certainly ways to make it work on the cheap — nuptials at City Hall, a backyard BBQ for reception. But keeping a wedding small means setting aside a girl’s dreams that she’s spent years carefully refining, from playing house to Pinterest.

Hey, that’s life; $30,000 can go for a better investment, like a down payment on a house.

Then there’s student-loan debt: My fiancée and I are among some of the first of our close friends to jump in the deep end not only because we’re sickeningly in love (as our friends regularly complain), but because we avoided mortgaging our futures for college. (I chose SUNY New Paltz specifically to avoid a big debt; he is lucky and talented enough to have gotten scholarships.)

But that’s unusual: A couple our age who both went to private universities could owe $200,000 for school.

You can drown in numbers like that. They make you feel like you have to wait until you’re more financially stable. And having kids seems out of the question until things even out.

That’s not true for all: In a piece for the Independent Women’s Forum, Patrice J. Lee notes that marriage rates are rising among “college-educated and affluent women between 25-34.” These women have not only finished their education, but solidified their careers — and now are rushing to wed while they still have time to have children.

But what’s an average, non-affluent lady to do?

Maybe that’s why getting married at 27 feels like we’re young, when we’re old compared to prior generations. It’s a common, sobering realization among my friends that, by our age, many in our parents’ generation had already been married, bought a house and had a kid or two.

Our parents notice the difference all too well, since we’re also living at home longer than they did.

The easier, cheaper alternative is to shack up. I know many couples who’ve lived happily together for years, with no sign of plans to get hitched. Actually, a recent Pew poll found that 39 percent of Americans feel marriage is obsolete.

I don’t buy that. I’m happy that, no matter what gets thrown at us financially, we’ll be tackling it together. It’s more than worth it.

Gabrielle Grilli is The Post’s letters editor.