Romantic/sexual feelings are usually associated with adulthood, but in
this case Mutsuki has the kind of crush kids very often have. The main
reason they wanted Sasaki to come back was so that he would keep him in
control, like how parents take care of kids. However, if Urie and
Mutsuki’s relationship were to become canon, the romantic/sexual
elements with it definitely would be associated with adulthood. I like
to think that mutsurie becoming canon would be the peak of their
development. (2/2)
Yup, that’s precisely it. It’s funny because early on in :re that’s exactly what I thought of Mutsuki’s feelings–that it was a typical student crush.
Adolescent coding quite involves a lot of exploration of where you fit in terms of relationships with family, friends, and yes, often romantic and sexual feelings–Touken, Akiramon being examples. They don’t get really explored (they’re implicit but not explicit) until :re except in Akira in TG, but Amon shuts her down because he is still a child and not ready for that yet. Shuu can’t express his feelings for Kaneki in TG beyond “I want to eat you” which is very childish.
But in :re, Touken, Akiramon, and Seiaki get focus and we also get focus on Ayato’s feelings for Hinami and Urie’s for Mutsuki and Mutsuki’s for Haise… also Karren’s for Shuu, Shuu’s for Kaneki, Ui’s for Hairu, Hairu’s for Arima, it’s a romantic disaster. Hinami’s feelings for Kaneki though not romantic also fit with the relationship-figuring-out in that she’s clinging to her older brother and his opinion of her, rather than finding out and being secure in who she is.
The difference in ships I consider likely endgame + alive (Touken, Akiramon, Mutsurie, and Ayahina–Seiaki is also possible) is Touken and Akiramon became canon while they’re still very much adolescents so they’ve gotta work that out as couples. Mutsurie and Ayahina I think are more likely endgame as in the end of their journey/character arc endgame, part of realizing who they are and becoming adults, or so I hope. The biggest obstacle in them getting together is Mutsuki and Hinami’s feelings for Kaneki, though Hina’s aren’t romantic but they’re still an obstacle, annnnd Ayato and Urie’s not verbalizing what they feel. Ayato’s further along in that he’s at least realized the limits of physical strength whereas Urie knows that now but lets Saiko act for him. It’s also maturity at the heart of it: speaking what you feel and understanding that physical strength can’t save you all the time is an adult perspective.