When it comes to relationship/marriage is a sex a "duty"?

My mom and I were watching Sex and the City the movie where Miranda's husband cheated on her b/c they haven't had sex in 6 months,and my mom said you don't sleep with you're husband in 6 months of course he going to cheat,so is she right? Do wives need to sleep with there husbands on a regular basic? Is it a "duty"? And I know that not all men are cheaters.