You have to wonder if this behavior by the gods was partly inspired by young women getting pregnant (oops) and, when asked who the father was, quickly said, "Ah... it was Zeus! Yes, that's it. He came in my door as a wolf and ravished me." This way she wouldn't be forced to marry the real father.
A lot of cultures have this folklore about women getting pregnant by sleeping out under the full moon, or eating taboo food or something. It might be a way to deflect blame.