And now that you know where to start, let me tell you where to end.
You do not want to read "The Mouser Goes Below," the last F&GM story Lieber ever wrote (to my knowledge). I don't know what happened to him as he got older, but the final story is dreck on a truly monumental scale. It's literally nothing but an excuse for the character to travel around the world, spying on all the various women he and Fafhrd have slept with, and watch as they have sex (usually lesbian sex) with other people.
It's not explicit enough to serve as true porn, if that's what you're looking for, but it's far too focused on the sex (and pretty much boring as all get-out) to serve as much of anything else.