please shii still cheated on tamaki despite the relationship being born out of a bet. they are still in a relationship that shii started. shii's priorities are all about having sex with tamaki that he would seek it on other people. it kinda cheapens his feelings for him.
If you’re talking about Shii meeting up with the guy that looks like Tamaki that was when he was in junior high before he got into a “relationship” with Tamaki. At that point he was only aware of his feelings and knew he didn’t have a chance so he faked his age and hooked up with a university student. I’m pretty sure they haven’t met up since shii and Tamaki have started dating and all of those are flashbacks (unless they really did meet up, honestly I forgot bc it’s been so long) but from what it looks like Shii hasn’t hooked up w anyone even when Tamaki’s been ignoring him
Also not shii didnt prioritize sex it just thought that come to you if you like someone and sexual intercourse is the main topic in this manga considering how tamaki's mom had put trauma on tamaki with that issue. Shii actually care a lot about tamaki feeling he knew tamaki not gay so he never force his feeling for tamaki during middle school and he cares a lot about tamaki mental state rn.
kinda sad that its complete. wanna know more about their story if they indeed end up together ;-;