It seems pretty common. I read people ending relationships over it online. I see people breaking up over it real life. It happens within every class (poor, middle, rich). It is romanticized in the media.

It seems really pretty common. I have friends (both male and female) who cheat a lot and have never been caught. Which makes me think it's even more common than one might believe because only the ones who suck at hiding it get caught. Sometimes I think a significant or even most of the population just aren't cut out for monogamy.