A coworker once used “naked friends.”
Lovers
Orgasm friends.
I’ve heard “friends with benefits” used in several contexts. The one that I think of as unique (or nearly so) is where two people (I tend to think of them as being of different genders) have little to do with each other except to share an occasional sexual encounter but otherwise maintain a casual acquaintance. In my view the “friend” aspect is more of a euphemism with overtones of “somebody I know.”
But if the term is meant as a substitute for any non-married sexual relationship, then there may be any number of near-synonyms for it. Common law marriage and roommates are two that I haven’t seen mentioned thus far. But both of those terms suggest a more personal relationship and more of a shared living space than the FWB does. I see FWB types as living in separate quarters and getting together one time for conversation and another time for sex and maybe another time for both, but otherwise not spending much time together and not sharing all that much with expenses and chores and humdrum activities.
Especially if its you doing the sleeping. Tacky to kiss and tell.
If its gossip (also tacky, but useful to sometimes impart this information on the unknowing), I think ‘friends with benefits’ is probably best understood without being graphic.
Second vote for “Ummfriend”. It leaves open the possibility that they are merely a bit interested in each other rather than full on doing it on the side.
euphimisms, eh? Well, if neither party is married, how about Unadulterous Co-Conspirators?
Otherwise, it’s all in the inflection. I believe they’re good friends. knowing glance
This one comes in especially handy when you’re placed in the awkward situtation of being obliged to introduce your “um… friends” to each other.
Womb-Mates.
Intercasual Friends.
Groin frere
I like “Pelvic Affiliate,” which my fiancee and I also use for each other, in that “fiancee” seems pretentious (to us, at least), and “boyfriend/girlfriend” seems rather juvenile*
*not that there’s anything wrong with the term, but we’re in our late 30s