All of Heinlein's novels featured it to some degree.
At the end of "The Puppet Masters", everyone has to go naked all the time because it's reasoned that it's the only way to ensure a mind controlling bug isn't attached to someone's body. There was one portion in his "Time Enough for Love" where (if I'm recalling correctly) the central character spent quite a few years living on a colony with two lovers sans clothing. Especially in his later novels he had this thing for depicting characters going against social taboos - not just nudity, but polyamory, incest, homosexuality (at the time much more taboo than today), transsexuality, stuff like that.