Gosh, I hate to be snarky but I just had to adjust your post below to be more reflective of reality. The sad fact of the matter is that these men you are talking about are not tired of sex, just sex with their wives. It isn't easy to get excited about the same old thing.

I hear of PLENTY of married men who tell their wives they don't want to have sex with them. Many of my women friends are constantly complaining that their husbands are too tired or too out of shape or would rather watch TV than have sex with them. There is a growing discrepancy between 30-40 something women who take care of their health and bodies and care about their appearances and the schlubby, overweight and, often, unhealthy husbands who don't (these are the husbands who don't have affairs, they see no need to keep themselves in shape if their only reward is to attract their wives). These are 2 career couples married 10-15 years with children.
Is the reason that women want to have affairs that their partners don't respect themselves or their wives enough to take care of themselves?