From Wikipedia, the free encyclopedia - View original article
A sexless marriage is a marriage in which little or no sexual activity occurs between two spouses. The US National Health and Social Life Survey in 1994 (Laumann et al. 1994) found that 2 percent of the married respondents reported no sexual intimacy in the past year. The definition of a nonsexual marriage is often broadened to include those where sexual intimacy occurs less than ten times per year, in which case 20 percent of the couples in the National Health and Social Life Survey would be in the category. Newsweek magazine estimates that 15 to 20 percent of couples are in a sexless relationship. Studies show that 10% or less of the married population below age 50 have not had sex in the past year. In addition only 20% or less report having sex a few times per year to monthly under age 40.
Sexless marriages can develop over time from a range of possible causes. According to psychotherapist Tina Tessina, "the most common causes of sexless marriages" are that "one partner had their feelings hurt or got turned down too many times, one got too busy or neglectful, or one or both partners has a communication problem of some sort". Clinical sexologist Judith Steinhart notes that "Problems in a marriage [like] lack of trust, anxiety, misunderstandings, pressure from children, all can impact a couple's sexual patterns." 
Some couples may have sexless marriages because they have different work schedules or busy lives. For couples with children, especially young children, the demands of childbearing and childrearing can lead to stress and exhaustion. Fatigue or exhaustion can also arise from other causes, such as chronic fatigue syndrome.
Adultery can lead to a sexless marriage in two ways: it can cause the partner having the affair to have reduced sexual interest in their spouse, and if the affair is discovered, the "innocent" spouse may cease to want to be intimate with the cheating spouse. Sexual aversion or "a low level of sexual desire" including a lack of sexual vitality due to age, past trauma, partners' incompatible sexual orientation or, simply, one of the spouses losing sexual interest in the regular companion.
Sexual dysfunction or difficulty during any stage of the sexual act, including severe vaginismus or erectile dysfunction. Lack of sensations, desire or ability to achieve orgasm resulting as side effects from medication or illegal drugs. Some antidepressant drugs such as SSRIs can cause difficulty with achieving an erection or an orgasm. Sexless marriages can be caused by post-pregnancy issues and hormonal imbalances which can be temporary or permanent in nature or by illness of one or both partners that affect physical or psychological sexuality (e.g., clinical depression of one or both partners).
A marriage may also be sexless if one or both partners are asexual or if the couple mutually agrees to abstain from sex due to religious principles, avoidance of sexually transmitted diseases, a platonic basis for the relationship or the goal of avoiding conception. Other reasons for sexless marriages are resentment in the relationship due to an imbalance of duties, responsibilities (moral, spiritual and religious); incompatible ideal, spiritual, moral and behavioral aspects.
Some chronic marital conflict can generate a state of permanent hostility that prevents or blocks sexual expression. It's usually the partner who behaves in a passive aggressive way the one who blocks sexual intercourse as punishment for some imaginary or real slight received from the other. Partners then feel resentment because of the perceived rejection by the partner who lost interest in sexual communication. Loneliness, anger and self-esteem lowering are normal reactions by a person feeling his/her sexual human needs frustrated by the voluntary rejection from partner. 
Some couples may be married solely for legal purposes or tax benefits, i.e. what is colloquially called a marriage of convenience. For example, in the US a spouse is entitled to Green Card if married to an American citizen or permanent resident. Another reason for a "marriage of convenience" is the lavender marriage, which conceals the homosexual or bisexual orientation of one or both spouses.
Habituation can be an important factor as well. Frequency of intercourse tends to diminish over time, especially after 1–2 years of marriage. Sex takes place with the same person all the time in the same way. Novelty and interest can be lost, and routine may dominate.
If both partners so desire, they can make a conscious decision to step up the frequency of sex and even to schedule times for sex, to kindly communicate with another in and outside of sex, or to play role-play games, and even use toys, practice "dirty talk", have "candle-lit dinners", look for unusual sex locations, watch erotic films, arrange weekends away, share fantasies and much more. The idea is that having more sex increases interest in sex. "L'Appétit vient en mangeant" (fr) - appetite comes with eating.