The same way you explain adulterous protestant ministers. The same way you explain embezzling bank Presidents, Alcoholic surgeons, etc...
They are human beings with all of their foibles and frailties.
But each of them understands that marriage is a sacrament, and for a sacrament to be valid you must both the proper matter and form. If the couple are not one of each then you don't have the proper matter and do not have a licit or valid marriage.
But “adulterous protestant ministers” haven’t become an international scandal, now have they?