Frankfürt
You basically have two options:
1. Behind the Hauptbahnhof (main train station) is a Red Light district with several whore hotels. The deal is you walk the halls. Providers who are available have their doors open and hussle you. Prices are on the low side, but so is the quality. Quite a few of the guide books describe the area -- I think Lonely Planet has something on it and so does Rick Steves (or, at least, he used to -- which I found quite amusing.) You negotiate a fee -- when I was there three years ago I believe FS went for around $100 or less, everything covered, "wham bam thank you M'am." You can also buy the local tabloid and check the classified section, if you know some German.
2. FKK Clubs
Do a search on Google for "FKK Club" Frankfurt and you'll get several listings. FKK Clubs are legal sex clubs in Germany. They're sort of a cross between a health club and an orgy. You pay a door fee and are given the key to a locker. In a typical men's locker room you disrobe, and wrap the towel around your waist. (Some of the clubs have seperate lock boxes for your wallet, if you don't want to carry it around with you.) Then you go out into a common area which often includes a pool and or a jaccuzi. There are lots of naked women walking around. By law, only non-alcoholic drinks are served. Usually there's some porn video playing somewhere. The girls aren't usually allowed to hussle, so you survey the scene and pick whomever you want to chat with. Once you get acquainted, you can retire to a private room or begin fooling around out in the common area. Regardless, there's a 1/2 hour charge to spend time with each girl. When I was there three years ago, the door fee was $50 and the 1/2 hour fee was $50. The one I went to (I think it was called Paradeis, but I might be wrong) had a wide range of women, from German to East European to African to a cute Israeli). There's a men's shower room, often a sauna or steam room, sometimes a licensed masseuse is available for the equivalent of a session fee. Anyway, in my opinion, it's the way to go. You'll never experience anything quite like it. It's sort of like heaven for hobbyists.
Note: the FKK Clubs are generally located in industrial, out of the way areas. However, it's Germany, so you can get just about anywhere by public transit. I was lucky enough to find very specific info on a Web site detailing which train to take, which street to walk down. However, English is wide spoken and if they have a Web site, chances are there's a phone number -- sometimes even English instructions.
Here's a site with some listings in English, including prices, phone numbers, maps and driving instructions (very helpful), and a great FAQ:
http://www.kaleus-clubindex.de/kaleu/fkk-en.php
Surprisingly, you can also find current and useful info on the World Sex Guide forums. They actually have a section devoted to the topic. Go to:
http://www.wsgforum.com/
click on "Forum Search," type in "Frankfurt" and choose the "Germany FKK Club" section (the board's software is the same as this board's, so it will all be familiar). You'll find lots and lots of info.
You can also check the British-based Captain 69 Web site as a guest for other options in Europe. Some of the info outside of England can be out of date, but there are links. As a "guest" you can get lots of info except rates. But the rates are often on individual web sites anyway. Happy hunting:
http://www.captain69.co.uk/guest/