cheater repeater
a married or single person that is constantly involved with married or attached men..

The lowest form of a person that gets a sadistic thrill out of being the "other woman" or "other man"..

side note- they are most likely elitist, insecure assholes in all other areas of their life, too.
ex; "Is Jackie going out with someone's boyfriend/husband again?"

"Yes Jane! She is a cheater repeater!"