Um de dum... well, the only things I can think of off hand are Nutaku.net or the adult games on Newgrounds.com. None of which are MMORPGs. Some sims. Some puzzles. Some card based.
There was a game awhile back that was based on the Blizzard franchises StarCraft and Warcraft, but I can't remember the name. Bonecraft, maybe? Pretty sure they charge after you reach a certain point on your free trial though.
Conan... uh... I think there was more to the name than that, but can't recall what it might have been. I tinkered with that for a while. It was an MMORPG, and free to play, but I think about as racy as it got was letting the female avatars go topless.
Other than those options, like John988, I can't think of anything specific. Have you tried typing in "Free Adult Online Games" in the browser? When I tried, I got a long list of sites, but admit I didn't actually go in and give any of them a try since I'm nursing a five year old computer along for as long as it lasts and am too old and too tired to worry about the viruses and such potentially hiding in unknown sites.
As for WoW, I was one of the strange ones who loved it from Vanilla all the way through, but haven't been able to bring myself to play since mid October when the kids and I held an online wake for my wife. (And, yes. That wierdo
was me.) However, unless you hook up with the people who habitually turn Goldshire and Crossroads into brothels and ERP, there really isn't much "adult" or "erotic" about it that you don't bring with you.