involuntary threesome
when significant other tries to include himself/herself in the act of cheating
Jill: I'm gonna fuck someone else tonight
Jack: Bet you won't cheat on me right in front of my face
Jill: Looks like you're gonna be involved in an involuntary threesome
Jack: Bet you won't cheat on me right in front of my face
Jill: Looks like you're gonna be involved in an involuntary threesome