There was actually a point to Joseph's seduction beyond mutual pleasure. It was very easy to build or destroy someone's confidence in the bedroom. It was also one place Rhys had no experience whatsoever to convince himself he was lacking somehow before Joseph had met him. It did not take a degree in psychology to see Rhys' tendency towards self-sabotage, nor his clear trust and abandonment issues. The boy wore his feelings on his sleeve for all to see. It was part of the reason he'd come down on the sheriff so hard. By nearly ignoring Rhys, Amadeus had only reinforced all of the fledgling's insecurities and concerns.
Turning any fledgling into a functional Kindred required enormous resources. It was a full-time job, even for sires with large retinues and other childer to help. Amadeus had been right to save the boy and protect him from the rest of the Tremere, but he'd basically only made Rhys' insecurities worse by ignoring him until he couldn't anymore.
It was part of the reason Joseph had been so all consuming and protective. Because Rhys wasn't the one at fault and had been the first to try to be the adult in the room by apologizing for his fledgling mistakes. His elders needed to start at least meeting him in the middle.
Starting in the bedroom, especially with Rhys' current conviction he only cared to have sex with Joseph, worked quite well for a start to building his confidence in himself in general. The only input on the matter he had for now was Joseph's feedback. Which was of course positive not only because he wanted to make the boy feel good about himself, but because he was doing as he'd been encouraged and expressing and acting out his desires. And it was only his second sexual encounter. Rhys was indeed an apt pupil and learned very quickly.
Joseph was trying to draw things out, not only to extend the pleasure for both of them, but to encourage Rhys to ask for what he wanted and start to take the initiative himself. Hence the mercilessly slow pace of his feeding and sexual teasing. Teaching him to pursue his desires instead of merely letting life happen to him.
It was just possibly the most pleasurable way to conduct lessons in self-confidence and self-agency.