Hey there was a comment on here a while ago that explained their relationship perfectly. But I lost the link to it so I'll try to summarize it as best as I remember it.
In chapter 11 tamaki and shii essentially made a "contract" that shii would "force" tamaki to masturbate so that tamaki wouldn't feel guilty himself. You can even see it in the chapter when shii asks tamaki if he can french kiss him and tamaki tells shii not to ask him for permission/consent.
The best thing I could compare this to is the fact that SOME women like rape fantasies, sometimes because they feel guilty about feeling pleasure, so they like rape fantasies so they can blame their partner for feeling good. Essentially, it is "I didn't want to feel good, but he forced me, so it's not my fault I felt good." Here is basically the same thing.
Is this the best way to deal with trauma? I don't know, but the two characters consented to it so who am I to judge ╮(￣▽￣)╭
Imagine having a relationship with the person you like but you know that they just did it by force. I know that Shii is mean for forcing Tamaki and triggering his trauma but I want Tamaki to be stronger. I want him to face it more that’s why I don’t dislike what Shii is doing. I relate myself with Tamaki. I have lots of traumas but I am very afraid to face it and I always avoid it. That’s why I’ve never overcome it until now I also lost a lot of good opportunities in life. That’s why I’m kinda thinking that I want to see Tamaki be happy and overcome his trauma. Aaaaaahhhhh sorry I said too much nonsense stuffs.