I think this is a far more common thang than most of us realize. I'm not married, but a few friends are, and from what I've heard, the sex starts dropping off after about 3 years, or after the first child arrives. When I go out with my single girlfriends, we get hit on by guys in their 40s and 50s, and story is usually that they need some fun with a woman because the wife has stopped or doles it out sparingly. I don't know if its the same for women - but I suppose it is. I will not date a guy I know is married, no matter how sad his story is - I just won't be the other woman or a home wrecker. But a few of the older guys I have hooked up with have problems with getting a hard erection. They get semi or chubbies, and thats OK - they're hard enough to get in and feel nice, or blow, but the guys have real problems with self confidence. I have almost felt like a social worker with one of them - he needed major boosts to his self confidence and after a while, voila - his erections were improving.
I don't really know what else to say other than it seems to be a problem with boredom, maybe monogamy being unhealthy for some people, or maybe they just weren't compatible in the first place except for fucking...
If I ever get married and find my husband's cock having trouble when he gets older, I'll do my best to be a good lover.
Women have it so much easier than men when it comes to having sex.