Read a while ago and looking for this story again. Guy has a best friend and best friends wife it seems like has never truly liked him. At some point guy is spending a few days or something with his buddy and wife. When he’s sleeping his buddies wife has sex with him and it’s the best sex main character can ever remember. It happens maybe 1-2 more times I think? Basically by the end of the story you find out guy knocks up the wife, his buddy dies, and he marries the girl to raise his child with her.