Not much hope for MUDs or MUSHs anymore, really.
They form cliques and then start to die off.
There are also many people who inhabit MUDs that cannot type properly, even when English is their first language.
I recently started on a MUD called "The Gathering…" which, while it isn't sex-oriented, is adult-oriented and allows for sexual roleplay that's done out of the public eye save for one open area that, funnily enough, nobody goes to. Ever.
It's a nice enough place, but the people just can't help scaring away newbies. I was lucky enough to join while the mostly-antisocial crowd was on, while others receive the cringeworthy "SO RANDUMB WELCOME MESSAGE >x3 RAWRR CAEK" spam by the same 5 people who have clearly been there too long and have likely contributed to the MUD's downfall.