Most Helpful Girl
... because the man who cheated on us, though his actions were inexcusable, is also the one who we were (and probably still are) so emotionally invested in. Hence why not only women but also men who are cheated on usually channel most of their anger/frustration against the person who their s/o cheated with, versus at the actual cheater.
(Doesn't make much sense when you look at it without your sense of reason being clouded by emotions but.. there's a reason why managing to do something as seemingly simple as being in a lt, exclusive relationship has been such a struggle for mankind for millennia!)0