Well if you read the raws Sawa-san already gave the uke a hint that he and seme is in a relationship. Their relationship is not Unclear! Seme supports Uke on his dreams and Uke just pursues it. Now he is back ofcourse his bf will be there with him and it's CLEAR that Sawa-san and seme have a communication they even created a project where Sawa-san is the main ambassador for Seme's product.
Uke knowing Seme cheated on Sawa-san with him turned on his idiotic side and still keep on being a damsel in distress. We understand that seme doesn't love Sawa anymore why humor Sawa-san when he got back even go to a hotspring trip with him while the third party uke is at his house.
I don't get why others hate masato when he's clearly such a cute lil bean who's a victim to the unclear relationship between kuramoto and reiji. I'm gonna stay longer for this one. This is one serious and great mangas I've ever read.