Well, if Melian's girdle (waistband?) is not strong enough to prevent a Man from entering, what hope does it have against a rabid wolf?
But truth to be told, Melian herself foresaw the possibility of having a man entering to steal Luthien away... sounds very, very similar to some of the Greek/European mythology... I think, however, Turin's doom had something to do with Morgoth's curse, though, of course, Morgoth himself sought to bring the curse home.