Wow what a fascinating thread, I love to learn about psychology like this. Makes me want to write a romance novel where the couple conquers a sexual disinterest issue.
I don't think I really have anything to contribute - I did at one point decide I wasn't happy with the relationship I was in and had to take a firm stand and not give in when that person tried to tempt me into sex any more, but that's kind of the opposite thing, I wanted sex but was becoming somewhat repulsed by having it with him because I was realizing more and more that he was just the wrong partner for me.
Does that mean you find it difficult to experience sex purely as a physical sensation, like scratching an itch?