Heh, well, Heinlein's group marriages are kind of awesome (in his later books, he starts getting really self-indulgent with his crossovers, and pretty much all his characters end up marrying into the same group marriage. As do all sorts of crossover characters from other canons, and various RL SF people. Because they discovered a technology that lets them go to fictional universes. But only if they are already fans of it. And no, I am not making any of that up.)

But anyway, this being late-period Heinlein, his characters do have fairly intricate relationships within the marriage, but sex isn't really part of the variety. Because these are Heinlein characters. They do have enough time in the day to go to bed with all of their spouses (even if they have to invent time travel to do it) and they do. Regardless of whatever else is going on. Or whether it's really logistically/culturally/emotionally a good idea, because in these stories, it is always a good idea.

A story with a group marriage where everybody doesn't have sex with each other, and that is PERFECTLY FINE is deconstructing Heinlein already. :D