Shuu was never in love wuth Misaki in the first place. By Shuu's words, he loves Misaki as a brother and wants to protect him.
The night they did it, it wasn't because Shuu was in love with Misaki. It was because Misaki's meds didn't work and he was in a lot of pain, so Shuu did the next best thing.
Misaki too. He referred to Shuu as his "husband" but when they were having a conversation at the Houjou house, he thought of Shuu as a father and an older brother.
to did it with someone he thinks as a father and older brother ,okay it's fiction .
I understand, I just wrote this as a funny take away from the story.
Shuu did so much to help ML's family's situation and he still took his man at the end lmao. Like I get it that he just wants him to be happy and didn't seem to mind it but dang