shotacon
Since Otomo is already so obsessed with the Prince, it's easy to translate this into a secret attraction. He's very timid though, so he wouldn't admit it on his own. But there is a push, as Salde gets older, gets into puberty, and a flirtatious side of him comes out...Otomo is also very protective. He gets worried that the prince's newfound curiosity might get into trouble with a stranger! So, he indulges the prince himself. Better that it be with someone he knows, right?
He is a little unprepared for how forward Salde is, as he has never had a real sexual experience himself. They are both out of the loop, but Salde is given most of the control, doing whatever he thinks would feel good. Otomo can't help but find him attractive, the way his small hands rub him, and his sloppy kisses. He is happy as long as the prince is happy.