-.-

Dude they've been dating, read volume 8. Senpai is just a tsundere but even in this chapter he is saying and has said in his own way, that they are together already. I've read this over many times so I'd suggest re-reading to catch those details. He calls Morinaga family for a long time because he's special to him, and because he wishes to continue his life with him. Couples are family. Also, he didn't outright say no to the proposal. He is very literal, so he was more concerned with the fact that it's not possible for him like Tomoe and Kurokawa in America since it's not legalized. When you read in between the lines, he wants to say yes but knows it cannot be done in Japan. This is why he told Morinaga's mother he will make him part of his family. Since gay couples cannot marry in Japan sometimes they add their partner to their family either through adoption or a partnership system.

This level of tsundere is ridiculous and hurtful towards his partner. There's a limit. And we passed it volumes ago.