Avhaben,
I think you are misunderstanding the Gemara if you are taking it to mean that an official “marriage ceremony” (which does absolutely nothing from a halakhic point of view) that by itself violates no issur is worse then the maaseh atzmo which is a violation of one of the Sheva Mitzvos B’nei Noach (and would be yehoreig v’al yaavor for Jews).