Fusions aren't limited to romance. They are the physical embodiment of love, no matter what kind. Smokey Quartz was a result of the bond between Amethyst and Steven, which clearly isn't romantic (in my opinion, it's more of a brother-sister relationship).
That said, I do agree that it would be a bit weird to see the "new" Rainbow Quartz, although I think that's only because you have to take into consideration what the previous Rainbow Quartz was like…

Well then, Amethyst and Steven has had sex twice.