Basically I think that Fuuma & Subaru can be interested in one another and still be in love with Kamui and Seishirou respectively. And that's why I don't feel it's OOC. I can't see Fuuma and Subaru settling down in a long term relationship in the canon timeline, but I can see romantic elements as possible in any affection that develops between them. Fuuma/Subaru does not "relegate" who they really love, if written IC.