Yes, but I can also see possibility for S supports becoming the support that have pair ending in FE3H. And pair ending is not necessarily being romantic in FE7, FE8, and FE10 (some kind of pair ending). In another word, I don't think S support need to be romance exclusive.
Yes, same-sex relationships will be more limited numbers in my assumption. But for a company who is not famous for being progressive on gender issues, it is more concrete to assume that they will choose non-romantic pair ending rather than choose two once-married men for same-sex romance options. For example, I can totally see the case that only Linhardt and Mercedes are same-sex romance ending, and the others are all story-based pair ending. I am not claim my assumption is better, as you can see the straight options are out numbered the same-sex options, but I think it's more reasonable.
By the way, I don't think that having non-romantic pair ending is to remove the romance of some of them, it depends. I don't see every FE need to have full availability on marriage like Fate. So having friendship pair ending whether same-sex or not wouldn't be a problem to me. Yes, no same-sex romance is a problem, but I won't say all same-sex friendship pair ending should be romantic. Lyn and Florina, Raven and Lucius, Marcus and Merlinus, Eirika and Tana, Ike and Soren, maybe some of above are fit to romance, but they also have reasonable story based on friendship. I just won't see them should be romantic.