As romances flower into long-term relationships, the sex commonly, cruelly, wilts. If once you couldn't get through dinner without a hurried tryst in the restaurant bathroom, now a few weeks, a few months, maybe even a year can pass without a roll in the hay, or at least without one worth remembering.That's all fine and well if both partners are happy with the new order of things. More often, however, vanishing sex lives cause couples distress — especially if mismatched desire leaves one person feeling routinely rejected, and the other incessantly hounded, perpetuating a nasty cycle of resentment.“It's not just about different sex drives, but a complete lack of empathy about being in the other person's shoes,” said marriage therapist Michele Weiner-Davis, author of “The Sex-Starved Marriage: A Couple's Guide to Boosting Their Marriage Libido” (Simon & Schuster) and founder of divorcebusting.com. “That sort of breakdown can put marriages at risk for infidelity and divorce.”Couples could save themselves grief if they discuss expectations for sex as they plan their lives together, just as they discuss values around children and finances. But people mistakenly believe sex is something that should naturally fall into place, Weiner-Davis said.“Sex is not a personal decision,” she said, “it's an interpersonal decision. And, like everything else, it has to be looked at as something that's nurtured.”Multiple factors conspire to diminish a couple's sex life once the idealized halo of new love fades away: a partner's annoying habits, job stress, babies, relationship strife, life trauma, depression, aging bodies, boredom with the same old sexual routine. Monogamy itself, for all its social and emotional benefits, contradicts our evolutionary instincts to move on to new partners. Incompatible sex drives, initially masked by the lusty bath of neurochemicals, might suddenly reveal themselves.It happens. When does it become problematic?There's a natural ebb and flow to any sexual relationship, so a dry spell doesn't have to be a crisis, especially if you can chalk it up to acute stress or a busy travel schedule, said Dr. Virginia Sadock, director of the human sexuality training program at New York University Langone Medical Center.But if those dry spells happen with some frequency, she said, it's time to pay attention.Sex is physically healthy, emotionally bonding and acts as a lubricant (not that kind) to grease the wheels of the relationship when life gets tough, Sadock said. Not having it not only might indicate a personal or relational problem, but it also denies a relationship nourishment.There is no prescription for an ideal amount of sex, but it's a primal need, “so if you're having sex less than every couple of weeks or a month, probably something's up,” said Dr. Gail Saltz, a psychiatrist and author of “The Ripple Effect: How Better Sex Can Lead to a Better Life” (Rodale). (The average rate of intercourse for married couples is 1.7 times weekly, but there's wide variability among ages and individual couples, and experts say every couple should decide what's best for them.)For many people, sex is a way to feel close, connected and desirable, so going without feels especially hurtful, Saltz said. It can be hard for the lower-desire partner to understand that need — just as it can be hard for the higher-desire partner to understand that less sex doesn't necessarily mean less love.Partners dig in their heels at their peril.Sex, Saltz said, “can be a testing ground of what you and your partner can compromise on.”How to wake up your sex lifeFor the lower-desire partner:Be honest about the root of the problem. People usually blame sexual disinterest in being “tired” or “stressed,” which often isn't the full story, said Marty Klein, a marriage and family therapist and author of “Sexual Intelligence: What We Really Want from Sex — and How to Get It” (HarperOne). Are you angry about something? Do you not find sex enjoyable?Get a physical check-up to determine if hormones, health or a desire disorder are affecting your libido, said marriage therapist Michele Weiner-Davis.Don't wait for fireworks. Whirlwind tear-my-clothes-off sex is rare later in relationships, so seize the moments when the embers of passion are glowing, if not bursting, and go for a romp, Weiner-Davis said. If it's still not happening, schedule it.Just do it. About half the population needs to feel some arousal before they can feel desire, Weiner-Davis said. This is also just a kind gesture to the higher-desire partner, as the lower-desire partner tends to control the sexual schedule.Instead of saying “no,” say when. Telling a partner, “Not now, but after dinner,” is received much better than flat-out rejection, Weiner-Davis said. Also, make it a point to initiate sometimes.For the higher-desire partner:During a non-sexual situation, discuss your concern in a non-attacking, collaborative manner, Weiner-Davis said. “I miss you physically, is there something I or we can do to make this happen?” If it's not working, see a certified sex therapist.Do the things that turn your partner on. That may not be sexy lingerie, flowers or anything that turns you on, but rather doing the dishes or encouraging a night out with friends, Weiner-Davis said. Learn your partner's love language (the way he/she feels loved): affirming words, quality time, physical touch, acts of service or gifts.Stop pursuing. Stop asking for sex or being available for it, go do something else, and let your partner come to you, Weiner-Davis said.Accept that your sex life may not be as mind-blowing as you had hoped. Our sex-crazed society makes people think sexual deprivation is worse than other disappointments, but it's not, Klein said.Rx for the couple:Have an adventure. Do something new and fun outside the bedroom, be it hiking or viewing an art collection, which can make you feel closer and pull you back in the mood, said psychiatrist Gail Saltz.Shake up the sexual script. Do it in a different room, at a different time of day, or scrap intercourse altogether and get intimate in other ways in the shower, said psychiatrist Virginia Sadock.Adjust your expectations. Older couples, in particular, can be paralyzed by performance anxiety if they expect sex to feel and look like it did in their 20s, so they withdraw, Klein said. Rethink the rules to accommodate less lithe and reliable bodies.Establish masculine and feminine energies. Sexual chemistry requires a dichotomy between the assertive, guiding masculine energy and open, vulnerable feminine energy, said sex therapist Marianne Brandon, author of “Monogamy: The Untold Story” (Praeger). It doesn't matter who plays which role. To cultivate the feminine side, which is often what suffers after a competitive day at work, activities such as yoga, dance or meditation can help.Imagine yourselves making love for the last time. It can help people rip through their defenses and get to a profound place of love, Brandon said.