The bathroom is basically a vacuum cleaner, that's how they solve that. As for jerking off, it's not allowed, but I'm 100% certain it's happened a bunch of times.

They usually don't have a lot of privacy, but there are some astronauts that have been up there for a long time, in times when the ISS was not very busy. For instance, Cassidy had the entire American side of the ISS for himself, and he was up there for a year. I don't think he made it through no-nut November.