Your statement sounds like you're saying all married men are cheats. But this is not true.
As for why some men do it, there could be many reasons for it...
- Loveless marriage.
- Lack of fidelity.
- Vindictive wife withholding sex.
- Women throwing themselves at the man.
- Boredom at home.
- etc.
I'm not justifying cheating. Rather, I'm merely listing what drives some men to do it.