It's a relatively old R18+ FGO Yuri doujin featuring Mashu x Ritsuka/Gudako,
Plot summary: After the end of ARC 1 story, Gudako retires from Chaldea, going back into society, eventually Mashu finds her years later having married a generic guy and drama ensues after the reunion where Mashu gets jealous that Gudako let someone else be with her and do R18 things, so she pushes Gudako down and forcibly have sex that eventually gets them to reconcile, in end Gudako divorces the guy by leaving her marriage ring behind and leaves together with Mashu


Anyone know the title? I think it's only in Japanese.