Yekke: That’s against the famous Ramban (and several other Rishonim) who point out that Ya’akov couldn’t be married to two sisters. And talking about Chiyuvim for B’nei Noach doesn’t help. We assume the Avos kept Kol Hatorah Kulah like B’nei Yisrael. So yes, it was before Mattan Torah so there’s technically no Issur. But the question still stands.
And the Midrash seems to look down on the fact that Shimon married Dinah (it says that they did a Ma’aseh that was done by the K’na’anim).