I've the theory that the myth/lie was invented to "chastise" women in their country. It is basically this version of "If you don't behave the way I want I would substitute you for one of those slavic women". The usual "warning" that you'll become a spinster since they would "get" a "mail order bride".

And after a while, they ended believing their own fantasy/lie, while the women around them rightfully suspected that the slavic women aren't that different from them and also want equal rights.

At the end of the day, the kind of men who spread this myth is the kind of men who preys on vulnerable women and manipulate everyone to get his way. So chances are that some of them know how false is the myth but stil want to use it/promote it.

But it is just a theory, I've no data to test it. XD