What about Riley and his vampWhores? Plus we have numerous references and featuring of it in Angel, he goes to a brothel twice and has various other encounters, the original script for season 1 had Kate working as one in her undercover role.
One episode where the theme is just a plot device to break up Riley and Buffy makes it a big theme that the show deals with? The only important reference I was referring to regarding AtS is Darla's background because she is a major character. When you have 100+ episodes of a a
noir show set in L.A., it's hardly surprising if an episode or two mention a brothel. By those standards pretty much every show on TV has dealt with prostitution, except maybe for children's shows. I bet it would be hard to find a show that didn't. Some of them even have prostitution as a plot in every second episode, like all the Law and Order and CSI shows.
Off the top of my head, I'm trying to come up with a show I loved that never featured prostitution as a theme in any way, and I'm coming up short.
Veronica Mars? Yes.
Six Feet Under? Yes, very much so.
The Sopranos? Of course.
Frasier? Yep, that episode when Frasier accidentally got arrested with a woman who turned out to be a transvestite hooker.
Homicide: LOTS, The Wire, Prime Suspect, Cracker, every cop show ever - obviously.
Battlestar Galactica - yep, we all remember Black Market.
Star Trek - yes, in every of the shows at some point.
http://en.memory-alpha.org/wiki/Prostitution
Twin Peaks - obviously.
Deadwood - DUH.
Hm, I can't remember if there was ever any prostitution on Lost... do we have a winner?