When Brandy Engler, Ph.D., started her career as a sex therapist, she planned to counsel women whose sexual desire had disappeared. But then, to her surprise, men started calling.

What she quickly realized: “Men have this whole internal world of emotions and experiences about sex,” Engler says.

But because guys aren’t exactly encouraged to spill their guts, sex becomes their surrogate for opening up.

That’s why she wrote her book The Men on My Couch, an insightful collection of case studies about the guys she sees in her practice. Here’s one such example—a case of passion fading from a relationship—and what you can do to bust out of the same situation.

Alex, a nerdy research scientist, was dating Kasha, a Brazilian woman with exotic, sensual features. Kasha swore there’d been passion in the beginning—she’d been attracted to the security Alex offered. But then, they moved in together, and the “mannequin sex” began. The passion had vanished. Kasha had traded lingerie for boxers and T-shirts. And she was cheating on Alex with a Russian man she described as assertive and intense.

When you move in together, a funny thing happens: The woman who used to be your lover becomes your roommate. Not exactly erotic.

“You see her in unattractive states—sick, depressed, PMS-ing in her ‘jammies,” says Engler. “There’s no mystery.”

This is when the “sexual intimacy paradox” sets in—as your emotional bond grows, your sexual desire wanes. So what’s the fix?

Act Like a Horny Teenager

“You have to learn how not to let her be your best friend, but a sexualobject,” says Engler. “And you have to be one, too.”

That sounds counter to everything women desire from a man. But when you’re in a monogamous relationship, women want you to view them with an “I-can’t-take-my-hands-off-of-you” gaze.

See if you can make mundane everyday activities sexier.

“If she’s just standing there washing the dishes, what would you do to her?” Engler says. “It doesn’t always have to be sex—it could be saying erotic things to her or flirting.”

This will take you back to how it was early in your relationship, when everything she did was hot.

Don’t Expect to Do It Every Day

If you initiate sex too often, you may end up with dull, obligatory intercourse, Engler says. On the other hand, if you give her some room to want it, you’ll have hot sex with a horny girlfriend.

“Let go of the expectation that a couple who is truly in love should have sex every day,” she says. “It’s about quality rather than quantity.”

If you ever sense she’s not into a romp, simply pull back and be transparent. Say to her: “I think we’re just going through the motions. What would really get you in the mood?”

“That would be incredibly relieving to a woman,” says Engler. It also forces her to take ownership of her desire—and lets her dictate the tone of your romps.