An older lady took it from you so why are you falling for another adult ???
I think you need to be abit more open minded - yes he was betrayed by an adult, but then he was betrayed by someone his own age too. then he suddenly finds an adult he can finally rely on and trust (Rio) - what's happening here is that he's confusing his feelings of trust for Rio as love, because this entire time he's only been trying to look for a lover in order to move on from his trauma - I think we need to be more patient and see how the manga develops and not blame Seto for his feelings! He's still young and figuring himself out.
A lot of the times, this can be a trauma response, I’ve seen girls that have been SA’d or harassed developing trauma from it and end up hyper-sexualising themselves after. It’s a response to the objectification they received during, and the mental damage done to them.
I don’t know if this is exactly the case here, but that’s what I’m getting from this.
This!! This is why I recommend the manga over the anime !! It took a minute but they're togetherrr