Why do single women knowingly sleep with married men?

I was reading about Newt Gingrich’s sordid past. His latest wife (his 3rd) was a staffer that he had an affair with and left his 2nd wife for. He left his 1st wife for the 2nd. His 3rd wife is supposedly a devout Catholic (Newt has converted).

Now why would a women who in principle thinks adultery is wrong get involved with a man she knows is married? Is she just stupid? Or do some men have some sort of evil seductive power that allows them to lead otherwise upright women astray?