You dont think they have an agreement of some sorts where 1 can fuck his own girl and dudes girl without dude present as long as dude can fuck dudes girl and his girl without him there? Really? Because I notice that a lot. Forestfaye& naomi, VicLit&Rybby etc..
And I dont think the dudes are making these deals themselves, they are making the deals with their own respective partner (the females) who are then bringing these deals to each other, probably saying something about how she convinced her dude to allow her to have a threebee with other chick and her dude but only if other chick has one with her and her dude. Its so common place that its basically in the unwritten book of the road. You get a ride, its expected.