Third guy has a crush on uke, seme lets him fuck his uke. Third guy ends up in love with both.
Hot DP, feminization
Uke loves to play games with seme and toy with their relationship, so seme ups the game by cucking the uke
Polyamory. Guy with unrequited crush on his friend was sex buddies with another friend for years, the friend found out about their sexual relationship, 3-way relationship forms
Two kouhai semes fighting over senpai uke
Cucking in Chapter 3
Fate/Zero dj - Fuck/Ero