it can be used as a form of self harm as well, Slen hated toguto back then but he probably hated himself too
Hmm, view it as another way. They don't do it to "Get back" at someone. Definitely lots of miscommunication is part of it but the main thing is the "sleeping" party doesn't want to be put on a pedestal. They want to show their partner that they are not perfect and to accept them as an imperfect person. But of course, we still want to impress our partners so at the same we want to be perfect as well.
In this particular story, sex was chosen because deers can only "mate" with their royal partners. Anything else Slen had done before didn't have as much of an impact on Toguto as his sleeping around. Which is why he stuck to it. In his mind, toguto accepted him as a "deer" only and he wanted toguto to accept him as "Slen" not the perfect "deer" of himself.
Idk why they always sleep around as a way to get back at someone and then complain about being 'dirty'
Like why was sex your only option if you were going to hate yourself so much for it? If the person you wanted to punish was him why are you punishing yourself too? It always annoyed me