1. Women still find married men attractive.
2. Married men are usually more capable of catering to the material needs of multiple women, than younger, less capable men.
3. If No. 1 is true, and No. 2 is true, then extra marital affairs are bound to occur.

Whether a man is promiscuous or not doesn't matter.
What matters is that he likes women in the first place. It's then a matter of, if those women find him attractive, and if he can add another woman to his provision sphere.

It's not a "men" thing. Even married women find other men attractive.
But the clauses for extra marital affairs on their part are different.
All in all, it's discipline and respect that makes loyal partners.

Analogy:
To lick sweet, you need:
1. Availability of sweet
2.A working tongue
3. If you have 1 and 2, then you can lick sweet
The answer to "Why can't you stop licking sweet?" is because 1 and 2 exists.
If there's 1, but no 2, you can't lick sweet.