Not, specifically, for mushrooms, but as a result of seeing "Spirited Away" which had a vital service provided for the spirit world, my brain somehow got twisted around and is now wondering why you never see a fantasy brothel as a setting.
I don't mean a purely humanoid one, either, since various parties in various fantasy novels occasionally wander INTO brothels. I mean one that caters to the weird things. Where does the Unseelie Court go to unwind?
Warning: Somewhat Disturbing Musings Behind The Cut
Nor would it neccessarily be all quasi-hentai and twisted either. One could make a case that the weirder creatures might not neccessarily even be into sex in the conventional manner. If you're an Ahuizotl, for example, an Aztec critter that drowns its victims and steals their finger- and toe-nails, you might find watching someone get a pedicure to be the absolute height of erotica. If you're a Red Cap, maybe what you want in a woman is somebody wearing the right shade of scarlet who can converse eloquently on the subject of textile dyes. And of course, you'd need to keep virgins on hand to give the unicorns hand jobs...(okay, yes, part of it would be neccessarily perverse. It's a freakin' brothel.)
Some critters would probably be easier than other--selkie women may think human men are hawt, and just want a pleasant evening with somebody who won't be lookin' to steal their sealskin and trap them on land. I don't know what may float the Nuckalavee's boat, and I'd be afraid to find out--I suspect you'd have the Really Freaky Section of the brothel behind locked doors, staffed by weird, quiet women whom the other girls treat with pity and alarm. And guards. Mute guards of peculiar species for that section.
Ghost girls. Heroines who failed their fairy-tale quests and wound up here, while their true love married the woman made of straw or wood or rose petals. Golem girls created by artificers with a perverse streak (who might be good behind those particular locked doors, as an indesctructible and obedient automaton might be just what you need for the Nuckalavee...) Fairy changelings who didn't get discovered until too late, and grew up with the humans, which is the fairy equivalent of being raised by wolves, and are looking for someplace to belong. God! So many options!
It seems like the sort of setting with a lot of potential drama--I mean, sure, there's a "Best Little Whorehouse in Fairyland" vibe, but with a setting like that, you could hardly avoid some obviously dark and creepy and disturbing stuff going on. What the hell happens behind those locked doors? The tragedy! The pathos! Potential for admittedly raunchy humor, too, though--do you have to don a biohazard suit to jack off a cockatrice? (There are no female cockatrices, I don't think, so presumably this is the only option available to the young cockatrice with, y'know, needs.)
I am really taken with the notion. Granted, if I drew it, it would probably be a soft R at worst, since I'm just not that big into drawing PORN!PORN!PORN! and like I have time for that kinda thing, but still...sorta twitching in my brain.