Why do women cheat on their husbands/boyfriends?

What is it that makes women cheat on their partners? Why would they entertain that idea and go through with it if you cared about someone? Is it just a case of being too weak to resist temptation? Or the thrill of having great sex with someone when it's considered wrong? Or do they just not really consider it a big deal and it's something they can live with? This is a place of non judgement and I am just curious to know the mentality of how a person could have an affair or cheat on someone.