To be honest, while they might not be "we are a permanent couple", they are still together. They have gotten together. They don't know what the future is, and Seth does seem to feel unworthy, and Horus wants more than they have. But people acting like they are not together doesn't make sense to me.

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.