It's been a long time since a manga (h or non-h) made me feel sad, lol ;w;
According to the artist, the story is about immaturity (and maturing). They're (best) friends since a long time ago. The mc lives in Tokyo and fmc lives in Osaka but she went over there for the time being because of work. She has a boyfriend but isn't so sure about their relationship so they ended up having sex when they met again. They keep seeing each other from time to time but one day she tells him her bf proposed to her. She expects some reaction from mc but he only congratulates her. Both of them were already "adults" but still didn't know how to convey their feelings in the end and just decided to keep moving forward in their lives. That was their first step to maturing, I guess...