JayMatt19,
I looked at the Gemara a while ago. Contrary to the way it is portrayed the Gemara does not say that the goyim subsist on the performance of 3 of the 30 mitzvos they were mekabel. Read it carefully. The Gemara is interpreting a posuk. The first pshat is that the posuk is referring to the 30 tzaddikei umos ha’olam upon whose zchus the umos ha’olam are miskayem. The second pshat is that the posuk refers to 30 mitzvos that the goyim were mekabel of which they keep 3. In this pshat there is no mention of the umos ha’olam being miskayem in their zchus (and the contrast with the first pshat is m’duyuk fakert).
Additionally, even if the Gemara actually said what people claim it would not mean that preventing same gender marriage is more important that preventing same gender relations. Rather the Gemara would be saying that despite the goyim’s aveiros they are able to still exist on the zchus of a very small act of not writing a kesuba on these relationships.
This is like the Aggada that says that Nevuchadnezzar Harasha was allowed to conquer the world in the zchus of taking 3 steps to call out to a messenger for kavod shomayim. It doesn’t mean that somehow taking those steps was a zchus greater than the numerous aveiros of shfichas damim and avoda zara of Nevuchadnezzar, but rather despite those terrible aveiros Nevuchadnezzar still gets some schar for taking the steps. But obviously his schar would have been much, much greater if he would have not murdered, pillaged and served avoda zara.
So too here. While the umos ha’olam get schar for not writing kesubos on same gender unions, they would certainly get much more schar if they would avoid such relations all together. And if for some reason two men would have a celibate relationship but chose to write a kesuba for it, then that would be a much bigger zchus than that of not writing a kesuba.