Derech HaMelech – “I don’t understand why there is so much emphasis on the actual “marriage”. Regardless of whether they are “married” or not they will still do their aveiros. The “marriage” isn’t a heicha timtzah to it. Why should it matter whether they are allowed to get “married” or not?”
2 Reasons:
1. The Talmud says that this is one of the crimes for which the Great Flood was visited upon the world and that one of the reasons Hashem doesn’t bring destruction to the world is because Goyim who are Toeivas don’t write Kesubos one to another. (This means legal marriage!) Are you in favor of destruction of the world?