when everyone in one given group of people is dating or hooking up with exactly one person in another given group of people, and vice versa. in principle this is an analogue to a love triangle (whose terminology is mathematically incorrect)
Roommate 1: I noticed you never came home last night..
Roommate 2: Yeah I hooked up with this girl. Her roommate's hot too.
Roommate 1: Oh shit.. did you try to have a threesome?
Roommate 2: Hell no, I'm gonna introduce her roommate to you, then we can have a love bijection!