Research indicates that over 55% of married women are not interested in having sex with their husbands. I've worked with many men who also are not interested in sex with their wives. The problem is generally not a lack of sexual desire - it's that they are not interested in sex with their partner.
Sexuality in long-term relationships is the result of loving energy flowing between two people. If something is blocking this loving energy, the sexual energy between them often gets blocked as well.
There may be many reasons for loving and sexual energy being blocked, but the most common is what I call the "pull-resist relationship system."
Here's how it works:
One partner, I'll call him Bill, 'pulls' on his wife, Jan, for time, approval, attention, appreciation, as well as for sex. Bill may pull with niceness, caretaking (giving in order to get something back), gifts, withdrawal, anger, blame, or threats. These behaviors are a 'pull' when Bill is coming from an empty place within, a vacuum-like black hole that wants to get filled through approval, validation and sex. In fact, sex may be the main way, aside from work, that Bill's worth as a man gets validated and his inner emptiness gets filled up. It may be the main way that he feels loved.
Jan, rather than feeling loved by the niceness, gifts, or withdrawal, anger and blame, feels objectified. She feels that Bill is being nice or angry to manipulate her into having sex - not because he genuinely wants to give to her and express his love for her, but because he wants to get love from her. He comes to her like a needy little boy, wanting to get validated, filled or released. She ends up feeling used and drained when they have sex rather than loved. Because she doesn't want to be used and controlled by Bill, and because she is not attracted to him when he is being a needy little boy, her whole body goes into resistance and she no longer feel sexually attracted to him. Of course, it could be the other way around, with the woman pulling and the man resisting being used and controlled by her. The same pattern commonly exists in same-sex relationships as well.