According to a study from the University of Texas at San Antonio, it might be their hormones leading them astray.
The researchers used actor Ryan Gosling as an example. When the test subjects were ovulating, they tended to be more attracted to chiseled good looks like his and to see such men as good husband-father material, even if the men were really cads.
Not only that, even if they knew the man was a bad boy, they thought he could be reformed.
We should probably note that as far as we know, Gosling is not a cad.