The guys friends and friends wife introduces him to her sister. They get married, but she leaves cause she thought that he married her out of pity. He meets his ex-wife again in a gangbang. The guys there looks similar to him. His friend tells him that she still loves him
I think that is a Just Plain Bob story. Unfortunately I don't remember the name and since he "only" has 765 stories, I can't easily figure out the name.
I think the story you're after is 'A New One' by, as Sandtiger said, JPB.
https://storiesonline.net/s/67632/a-new-one
