Hell is other people.
No Exit probably ended with a threesome. I mean it's not such a far stretch...
You put a hot, nervous, clingy woman in a scorching room with a lesbian and straight guy who are both vying for her affections for all eternity, and someone's going to suggest it, probably Garcin, and it'll be a joke (sort of) at first and both girls will recoil. Then there'll be an awkward silence, maybe for a millennia or two, but on a long enough timeline, everyone goes bi... or at get horny. And if your choices are masturbation or threesome you're eventually going to get curious. Besides, there are no consequences in Hell.
Now I'm sure the management thought of this (as it's probably been happening room to room), so they probably have a safety or something... but what about the human spirit? Sooner or later our Sisyphus rolls the rock of women's chastity and monosexuality to the top of his hill for his moment's respite.
Observe the final line:
Garcin: Well, well, let's get on with it...
(might as well have been "let's get it on")
It seems Sartre's editors forgot the rest of the line: "bow chicka wow wow..."
"One must imagine Garcin happy."