I've read somewhere that God gave the law of divorce to the Jews but not to the Gentiles, quoted some rabbinic opinion about it, didn't say which one. Is there anything to that effect in the Talmud? I would imagine that to support such a basis would be that if there is no official marriage for Gentiles according to the Torah, even if countries established their own marriages it would make them unfeasible in the matter of divorce, which was not something given to them as it was with the Jewish people, I don't know if this logic of mine would have any merit since relations between Gentiles can be created and broken at any time according to great Jewish authorities but the question of institution remains, would Gentiles be appropriating a Jewish heritage by instituting marriage and divorce or do the Torah and the sages recognize that these things predate the Torah and in many peoples?
Although the concept of betrothal does not apply to Gentiles the way it applies to Jews, nevertheless, it is still considered a sin to have a relationship with a Gentile woman who was betrothed, although it is not punishable by death - this can be learned from the fact that the Talmud (Bava Basra 16b) lists this act as one of Esau’s sins - even though it took place before the Torah was given, and there was no official law of betrothal
Regarding the question of it being considered “appropiatian”, Maimonides (laws of kings 10:10) writes that a Gentile may take on any Mitzvah that he wishes (aside from keeping Shabbos, and studying the laws that don’t apply to him)