It could be his way of holding the power in the marriage. I've seen it done once. I can't believe any woman would get herself in THAT situation, where she wants it and he doesn't, but I have seen it once, and he was doing it to punish her. She didn't like sleeping alone, etc. Of course, the way to fix that is to stop asking for sex from him and in fact to give it a rest even if he tries That will put things back into perspective for him.
However, this may be a case of him having an affair, as I've heard that some men do that during an affair. OR it may be that he is having impotence problems, but is able to masturbate successfully, which is probably the most common reason why this would happen. I had a "boyfriend" once who didn't want to have sex, and I had simply never encountered anything remotely resembling impotence before and I thought he was (and he may have been) having sex with other women, so it took me years to understand he was having impotence with me BECAUSE he liked me so much and because I was a strong presence. He would have sex with really drunk short blond girls, because he could deal with that. His problem.
