My theory:
Men naturally lose interest in screwing the same woman the same way all the time. Once some of that "newness" is lost, they don't put the same effort into chasing their wife/girlfriend.
Women see the diminished pursuit, and correspondingly lose interest in sex. Also, some women feel that after moving-in/marriage, they've "sealed the deal" so they don't have to "put out" as often anyway.
So there they go... guy has diminished interest and doesn't pursue like he used to. The woman becomes less sexual and doesn't feel compelled to change that either. So those killer daily sex-in-a-strange-place sessions get traded for scheduled sex every 10-14 days.
And everything is nice and stable, until...
That little hottie from the gym pays some extra attention to your guy. All of a sudden, those hormones get all woken-up and he thinks "hey, a female wants me!". Then, over the next two months, she starts asking him for advice in the gym. She asks for "spots"... oh... and then they grab a bit after their workout. Next, she mentions that she and her girlfriends will be running around some next Friday night and wants to know if your guy can "break free". Guess what? He can! And the rest is history...
Now, how do you break the cycle? I'm gonna get flamed here, but the answer is for the woman to BECOME that girl in the gym. Do something different... change your appearance a little... break a routine. Then, use a combination of compliments/attention/sexual behavior to get your guy chasing again. The same biological mechanism that makes guys "tired of the same old pussy" is easily fooled. An outfit (i.e. schoolgirl, prostitute, naughty nurse) often bypasses the system.
Now, if the woman says "Hey now, it's not my job to keep him interested. He has to do his part!!!!", then she'll be correct. That, however, does not change the fact that the little gym bunny is coming... in the gym, or at work, or at your son's baseball practice, or in the church choir... she's coming, so it's just a matter of time. As the woman, you'll be correct -- all the way up to and through the divorce proceedings. Now, if you'd rather be right than married, then just stay the course