There are a number of reasons a person might lack sexual desire for their spouse. When you consider that sexual desire starts with what happens between your ears and then is greatly influenced by the marriage relationship itself, it is understandable that most of what is going to stall desire will be personal or relational in nature. There are, of course, some physical problems that can affect sexual desire too.
The following list can be used to identify problems or potential problems.
poor self image, poor sexual self image, inhibitions - Our culture and, sadly, the church have played havoc with how we see ourselves and our sexuality. Feeling bad about how we look or how we might perform sexually can fill some with dread that steals sex drive. Being ashamed of our body, or feeling one part of it is too small or strange looking, is counterproductive to sexual desire.
fear of intimacy - wounds from past relationships can be carried into present ones, making it difficult to desire intimacy and oneness. Men may fear intimacy because they think it's "weak" or unmanly.
childhood sexual abuse, molestation, rape - In order to dull the pain, fear and shame associated with a previous sexual experience(s), many victims repress or fight their natural sex drive.
lack of privacy - living in close quarters with parents or (foster, natural, or step) children. This is more likely to affect women than men.
guilt from (false or genuine) sexual sin - guilt over masturbation, playing doctor, promiscuity, abortion, premarital sex with your spouse, non-marital sex before you met your spouse, viewing pornography, adultery, etc. can make approaching sex very painful. Growing up in a strict anti-sex household can make people feel guilty about normal and natural sexual thoughts and desires. Some even feel guilty about their desire for their spouse. Still others feel guilt over things they want to do with their spouse; things they themselves believe are wrong or "kinky," or believe their spouse would consider weird or sinful.
busyness, stress, anxiety - it takes a certain amount of time and relaxation to make sex work. Always being stressed out or having too much to do will eventually wear out your sex drive.
depression - depression puts the skids on everything in your life, including sex drive.
unforgivingness, deep grief, bitterness, fear, anger, hate - strong negative emotions steal emotional energy from the rest of your life. These emotions don't even have to be directed toward your spouse to have them affect your sex drive.
other outlets - Investing large amounts of time into work or being emotionally involved with other people (real or not) can tie up the desire and energy that you need for your spouse. This may sound fairly simplistic, but it represents a host of problems - workaholism, a too busy lifestyle, preferring friends (male or female) over your spouse, an over active fantasy life, adultery, romance novels, pornography, masturbation and other sexual addictions - anything that ties up your time and emotions to the degree that it drains dry what you need to emotionally and physically desire your spouse.
When lack of sexual desire is grounded in a personal or emotional issue, it is helpful to talk it out. Pray and seek out encouraging folk (your spouse, a friend or counselor) who will help you face and deal with the problem in an atmosphere of safety and understanding.
As the problems are faced and dealt with, the natural sex drive will begin to assert itself (or your natural drive can be more correctly directed toward your spouse). You may need to concentrate on your sexuality for awhile until it feels more natural for you.
lack of nonsexual intimacy - it's difficult to desire someone that is a stranger to you. Over the long haul of marriage, your sex drive needs something relational to work with (Paul is always saying that the time you spend in nonsexual interaction becomes the building blocks for sexual intimacy).
lack of sexual intimacy, sexual dysfunctions, frustration, disappointment - repeatedly being rebuffed sexually can emotionally, and eventually physically, stall your sex drive. The same can happen for repeated lack of orgasm, impotence, premature ejaculation, retarded ejaculation, or other disappointments in the bedroom.
poor sexual technique, lack of knowledge about sexuality - a lack of understanding can cause things to go poorly in the bedroom. This can open the door to repeated disappointment and frustration which can in turn cause a lack of interest. A lack of understanding of gender and personality differences can cause a good deal of friction in and out of the bedroom.
lack of trust, betrayal, adultery - intimate relationships need a certain level of trust and commitment to operate well. When one spouse has abused the trust of the other, desire for intimacy is diminished.
lack of respect, abuse, manipulation, selfishness - it is extremely difficult to desire intimacy with someone who does not show genuine love or who consistently diminishes your worth and value in some way.
boredom - Most of us wouldn't get excited about eating the same thing every day for years; sex is no different. Fear about what the other would think can keep these feeling from being expressed, and the boredom just grows.
Relational issues are a bit tougher to resolve as they involve two people, rather than one. But if both people are willing to work at it, difficulties can be resolved. Pray over your marriage. Read good marriage books and implement their advice or visit and learn from a happily married couple. Sometimes it is helpful to seek out a counselor to resolve particularly difficult problems.
medical conditions - anaemia, high blood pressure, diabetes, multiple sclerosis, and hemochromatosis among others. Undiagnosed thyroid disease is suspected by some doctors to be responsible for a significant number of cases of low sex drive.
medicine, medical treatments and drugs - alcohol, prescription drugs (including most hormonal birth control methods), and street drugs are probably the single most common causes of low sex drive. Chemotherapy, high blood pressure medicine, antidepressants, tranquillizers and other medicines and medical treatments can affect sex drive. It may be possible to correct much of this by changing medications and/or dosages, so let your doctor know about the problems. Alcoholism is a very common sex drive killer in men.
hormones - a woman's natural hormone cycle gives her periods of greater and lesser sexual desire. Hormones can also affect sexual drive during pregnancy, lactation and at menopause. Low testosterone reduces sex drive in both men and women, but this is actually rare in men.
exhaustion - being occasionally tired happens to us all, but chronic exhaustion means you need to check your priorities. Eat well and get adequate rest and exercise (cut back or cut out the smoking and drinking). In men exhaustion can impair erection even if the man desires sex; similar impairment of function is believed to occur in women who are too tired.
painful sex - infections, a poorly healed episiotomy, endometriosis, back problems and other conditions can cause sex to be uncomfortable or painful, making sex undesirable.
For health related problems, see your doctor! Change your lifestyle to take care of yourself and get educated about the physical / technical aspects of sexual intimacy (see the TMB Bookstore).
Now, y'all, do not use this list to blame or shame your spouse. Take the time to prayerfully look at what you contribute, both positively and negatively, to your marriage. Then look to see how you can help your spouse with what they bring to it also. Think: prayer, forgiveness, encouragement, creative ideas...