It’s a 3 part story, it’s told from the boyfriend’s perspective, but he’s completely unaware of what’s going on, his gf is seduced by their friends and he hears them having sex, but he’s sure it’s the other girl.
Sorry if this is a confusing description.
Source: reddit.com/r/Erotica/comments/tg797f/lf_a_story_a_read_here_a_long_time_ago_about_a