If your partner is not interested in sex, then you have to consider the possibility that he is having sex with someone else, but this is not always the case, it could be medical or psychological.

There are many factors that can cause a low sexual desire in men:
Age, hormone deficiency, pain during sex, childhood events, depression, anxiety, stress, medications or drugs, weight, body image, habituation to one's partner and unresolved relationship issues.

It is also possible that he is masturbating regularly and therefore is not interested in sex. It is also possible that he feels guilty about masturbating.  Negative attitudes about masturbation may result in negative feelings about all types of sex. These negative emotions can result in low sexual desire, erectile problems, and difficulties with reaching orgasm.

One solution to these problems is practising Sensate Focus, below is a how to.

The first step to practicing sensate focus is to set up the right environment. A couple might use candles, play soft music, take a bath beforehand and do whatever else makes them relax and feel comfortable. Couples should undress. One partner –who we will call the receiver – lies down on a bed in a comfortable position, on his or her stomach, back or side. The other partner will take the role of the "toucher". Later, the partners will switch positions.

The toucher begins by exploring her or her partner’s body. The "toucher" should not try to sexually arouse the receiver. The toucher should merely explore, touching many parts of the receiver's body, noticing the various textures and sensitivities. During the early phases of sensate focus, the toucher does not try to sexually arouse the receiver. This helps eliminating the element of performance anxiety and the pressure of reaching the goal of orgasm. The receiver should make suggestions to the toucher, telling their partner what feels good and what is uncomfortable. The toucher should also remember to ask for feedback from the receiver. Remember this exercise is about communication. After doing this sensate focus exercise for several minutes, the partners should switch roles.

The first few times that a couple practices sensate focus they should not stimulate the breasts, genitals or anus or attempt to engage in intercourse. Couples may also participate in simultaneous sensate focus, where each partner touches the other at the same time. Sensate focus can also be combined with a massage.

After several sessions of sensate focus, both partners are allowed to begin touching their lover’s breasts and genitals. As each explores the other’s body, good communication helps each learn what their partner likes best. Over time both will gain skill in stimulation the other in more exciting ways than they knew before doing sensate focus. These new skills tend to increase the sexual excitement of love making and solve the problems of becoming aroused and reaching orgasm. As each partner learns what is most pleasurable to their significant other, they can incorporate what they have learned into their sexual encounters on a regular basis—and continue exploring various sexual positions and types of stimulation.