Too Late for Sincerity

Yu and Ryosuke are colleagues who were once friends with benefits. The two lost contact when Ryosuke was posted overseas, but when Ryosuke returned to Japan, he got into trouble and had to rely on Yu... and for some reason, he ended up staying at Yu's house. He begins a strange life living under the same roof as his former sex buddy, a colleague, but a promise from the past keeps crossing Yu's mind...

"If you get serious, that's the end of it."

(Source: ShuCream, translated)