Seth didn't kiss Horus on a whim. He didn't do that because they were friends. He doesn't allow Horus to be as close as he is or talk about being worried about Horus disappearing on a whim. He's scared, but he acknowledges their closeness.

Do I think Seth would accept the label of "boyfriend" if he was told what that is? Probably not. Partner, maybe. Even assuming they aren't a couple in any sense of the term, I don't know what folks need to happen to decide they ARE a couple at this point.