The longer relationships last, the more other things will coincide with sexual attraction, for example if the person gives you enough attention or helps you run the house.
Look at the problem from a physical, psychological and social perspective. In order to feel like making love, there must be physical stimulation; this needs certain hormones and the right touch. Hormonal disorders, certain medication and depression can block this stimulation. So it is important to look at these possible causes of the problem and treat them.
Psychological factors also play a role. Stimulants that induce the desire to make love are: considering your partner attractive, an erotic stimulation, fantasizing about making love, and trying to make a pass at someone.
Factors that block these psychological stimulants are: finding your partner unattractive, having negative thoughts about making love, recalling unpleasant fantasies about making love, negative emotions, stress and fear. Pressing norms may play a role.
Ideas like: "desire comes spontaneously", "love play consists of intercourse" and "if you don't want to have sex you are not normal" play a role.
Social causes may be problems between the partners. Communication problems (one wants to sleep, the other feels rejected by this) may play a role. There may also be a struggle for power between the partners.