Uhh

I'm wondering if the people in the comments are all early teens..? I feel like there's a lack of critical thinking.

It was a bit parental at first considering the situation and how Ryou and Shunya were so in sync and felt like an older couple to Tsukasa, but I do feel like over the course of an entire half year that dynamic shifted a LOT. Tsukasa is an adult and he quickly shifted into his adulthood fully with the others' guidance.
People can fall in love in only a few months and that's just with dating, but living with someone every single day and sharing your entire life with them, it makes a lot of sense that you'd start to gather feelings, especially with a kind hearted person who is actively helping you. He also didn't just let go of Ryou like nothing, he found closure and Ryou had set the other two up to make sure they can move on together. Shunya clearly still had love for him, but you can love more than one person dearly and still be able to move on, just as many people do who have lost life partners and remarry.

I think it was a beautiful story and endgame was just as cute.