That's the thing that I don't get they REALLY think it's ok. I guess it's because most women feel that when they don't want to have sexual relationship with someone they simply wont, and they keep those other guys around to just stroke their ego. Problem is the moment the current boyfriend slips up and does something stupid (which we all do) she'll do one of two things, go vent to her cabal of female friends or go vent to her ex/fwb to try and better understand what goes through a mans head when he does the dumb stuff he does. Sadly sometimes no matter who she goes to could end up with her at least entertaining the idea of hooking up with the ex/fwb, either the cabal of female friends will do the old "He aint ___, you shoulda never broke up with__" or she'll go to her ex and he'll say "You deserve so much better than that, he doesn't know how much of a good thing he has with you.....If you were my girl"
Just to be clear I don't want it to seem like I'm woman bashing, because as I already stated men do some dumb ass things, but while we do it we at least KNOW what we're doing is dumb.