I stumbled across the red light district in Sfax almost by accident about two weeks ago. It was just at sunset and I was wandering through the medina and there it was. I scoped it out, then went back to my hotel and cleaned up a little (had just traveled into the city and wasn't feeling my freshest) before returning later in the evening.
It is about two alleys long with maybe 15-20 places to choose from. Most of the girls are quite ugly, but there are a few cute girls and when you find them they are very reasonably priced. I didn't want to overpay, so I asked a guy before I negotiated what I should offer. He said for a foreigner probably 10 dinar for the old, ugly ones and 20 dinar for the young, good looking girls. A dinar is worth about 80 cents at the moment, so I ended up paying about 16 or 17 bucks.
It helps if you speak some French although after we were done, my girl practiced her very meager English, so I'm sure you could get by without. As we started, she asked a couple of times if I was Muslim; of course I'm not and didn't look anything but northern European or American. She made an ugly face when I said that I wasn't. I was wondering what the hell being Muslim had to do with what we were up to, but it turns out she didn't care about my religion at all. What she wanted to know was if I was circumcised. Once she saw that I was, she cheered right up, seemed to enjoy herself (as much as they ever do) and wanted to exchange phone numbers, etc.
Of course I didn't give her my real phone number and chucked hers as soon as I left the place. If I had returned, there was another girl I wanted to try out.
Sex was missionary position, condom, with kissing but no tongue. She still seemed somewhat conservative about the whole thing so I highly doubt you'd get away with anything adventurous.
Here's how you get there:
Circle the eastern wall of the medina from the outside. There will be a set of gardens with benches and floodlights illuminating the exterior of the medina. About halfway along the eastern wall you'll find a set of stone stairs that lead to the medina level, which is higher on that side. As soon as you reach the interior of the medina, turn left and stick as close as you can to the wall when there are choices of alleys. It is quiet on that side, but doesn't feel unsafe. You'll shortly come across the district and it's obvious with neon lights on the interiors and obvious prostitutes standing in the doorways with men milling about.
The women don't seem to have any trouble propositioning an infidel. If you're lucky, you're circumcised. When they ask if you're Muslim, pantomime scissors and nod.