It’s the great irony of having children: The very act that launches you into parenthood is difficult to achieve — ever again — once your kid is born.

It’s like nature looks at you and says, “What? You got what you came for. Find another way to jazz up your evenings.”

And it happens to everyone: No matter how much your boudoir tends to bounce before Baby comes along, it slows to a sort of sad, silent stillness (sigh) once the diapers start flying.

“I can’t think of a single couple I know who hasn’t been affected by this issue,” says sex therapist Ian Kerner, a New York husband and father of two.

But he swears there’s hope. In his new book Love in the Time of Colic: The New Parents’ Guide to Getting It On Again, Kerner and co-author Heidi Raykeil say there’s no reason to throw your libido out with the baby’s bath water. “It really is possible,” they write, “to do the hokey pokey and keep up the hanky panky.”

What causes the sexual fizzle between new parents? Exhaustion. Stress. Mom’s hormones, and her tendency to devote every amp of energy and inkling of empathy to the helpless, gurgling humanoid in the bassinet, leaving none for poor, pent-up Dad.

Kerner jokes that a working title for his book was “What to Expect When He’s Expecting Sex,” because Dad’s urges — and Mom’s reluctance to have her arms, breasts, and hair tugged at by yet another needy family member — are a common source of strife.

I remember those post-baby months so clearly: Sex felt like another chore on my household to-do list, rather than the delicious indulgence it had always been. I was confused, disappointed, and embarrassed at having gone from fit and frisky to tuckered-out and turned-off.

Kerner says it’s just as tough on guys. He confesses to having been so hard up after his kids were born that he once became aroused while listening to his wife read the book Hop on Pop to their son.

He’s not alone. I recall my husband trying to woo me with, There’s a Wocket in My Pocket and I think we can all agree that when you’re hearing innuendo in Dr. Seuss verse, it’s been too long.

Here’s good news, though: Kerner says that by expecting sex, a new dad is actually performing a vital task: luring his partner slowly (if sometimes annoyingly) back into a strong, passionate couplehood, “a crucial necessity if they’re to flourish and succeed as a family.”

Kerner insists there are two ways to do that.

Wrong way: “Just saying, ‘Let’s have sex now.'”

Right way: “Communicating, building anticipation, helping a woman rediscover those feelings of wanting, helping her to feel sexy, contributing to all the new chores that go along with having a baby in order to help her de-stress.”

Pay attention to that last point. Along with all the other surprises of new parenthood, I was shocked (who knew?) to discover there’s nothing sexier than a guy who can swaddle, shush, and sway a newborn to sleep. Sorry, that’s hot.

As your kids grow, of course, new sexual challenges arise. We recently entered the Kiddus Interruptus phase and I can promise you that nothing quashes the libido like having to explain to your children why Mommy likes sleeping with “just her boots on.”

Once you’re past the baby years, though, the solutions get simpler. Communication? Anticipation? No need when basic hardware does the trick.

Says Kerner, “We have a lock on our bedroom door.”