Novels like After I Turned From O to A I Became the National Male God or Transmigrating Into The Heartthrob’s Cannon Fodder Childhood Friend
Does Transmigrating into the Reborn Male Lead’s Ex-Boyfriend count? Spoiler MC thought ML was the bottom (so he'd be the top), but then in the bedroom (?) he later realized that ML was actually the top... and therefore MC would have to be the bottom.