"Eight is Enough" S2.E3 Triangles

Tom takes Nicholas to the local indoor pool to teach him how to swim. They of course remove all of their clothes (swimming indoors in the nude was common for men and boys in the early 20th century) only to run back to the locker room when they learn the pool is coed at that time. Wikipedia says that the practice of men swimming nude had ended by the early '70s with the rise of coed athletic activities, so people watching in 1977 would have gotten the joke even if it seems bizarre now. Never shy about recycling jokes, the show later had Tom taking Nicholas to a "Snow White" movie that turned out to be a porno and David and Nicholas touring an apartment complex that turned out to be for nudists ("I thought going natural meant they didn't allow any junk food!"). In every case, the two enter, followed by a reveal of a sign explaining the true situation, and the pair running back in shock.