Resentment/hurt feelings such as;
i) She has hurt him with comments, and she may not even know she has done so. She has told him he was a poor lover he feels his sexual performance has been criticized, or she compared him to another partner, the latter is a particularly bad move.

ii) He feels he isn't a priority for her, or she isn’t doing what he wants her to do, this often ends up being something in a more traditional realm, things that make him feel he is important to her, such as cooking dinner for him.

iii) She made a decision on her own that would affect both of them and didn't tell him. iv) She has trapped him with sex, got pregnant without his being aware. And sometimes in the last case this will lead to men having sex and not being able to ejaculate.

She is no longer sexually attractive to him. This may be because of physical factors - because she has gained a lot of weight and doesn't look good to him anymore, she doesn't smell good to him, isn't clean about herself or because of social behaviors, she drinks too much, says or does inappropriate things and embarrasses him. In both cases she has become a woman he isn't proud to be seen with.

She has had an affair and he found out about it. For most men the thought of another man’s penis inside of their wife is one of the most crazy making and upsetting images they can imagine and believe me they do imagine it. It is as if they have a built in personal video in their heads and that image will be on an endless replay loop.

He claims to have a low sex drive or he has no libido that sex simply doesn’t interest him. This is rare but possible. The no interest behavior is invariable as the result of something else.

He is impotent and doesn’t know why or how to address it and buries his anxiety about it by becoming asexual and rejecting any overtures from his partner. Impotence has made many a man a workaholic as he channels all his energy into producing.

Has he experienced bouts of impotence? If so he may not want to talk about it, or he hasn't told you and the more you pressure him for sex the more stressed he gets and that feeds on itself. Is he diabetic?

Has he had heart or blood pressure problems and is on medication? If he is on any medication it may be causing impotence? All three of the above can create forms of impotency.

Ask a pharmacist to check for possible side effects of any medications whether prescription or not. They will be listed in the PDR, (Physician’s Desk Reference). Smoking and diet can also impact potency as well.

He has other interests – fetishes that he doesn’t feel he can share or that she won’t entertain and he needs those to be sexually stimulated.

Boredom factor. The sex is always the same and he would rather do himself than have to do the same thing again.

There is someone else.

They have realized they are gay.
In conclusion there is a lack of information for me for to be really accurate for you. So what I have spelled out are the reasons men have shared with me why they are not sexual with their wives. Your options are to have a non-judgmental conversation about this with him. Often more difficult than people know. You might consider printing off the reasons and asking him to read it privately, not while you watch him, and ask if any of these ring true for the two of you. Or you might seek out a good counselor. AASECT (American Association of Sex Educators Conselors and Therapists) has a listing of AASECT certified sex therapists in your area. http://www.aasect.org

Good luck.
Warmly,