3rd Rock from the Sun - Season 5 Episode 19 -

Frank, a former student of Mary's, returns for a lecture series and makes Dick (unaware Frank is gay) jealous. When Frank confesses he has a secret, Dick jumps to the conclusion that Frank is also an alien, ""comes out"" to Frank, and is soon cavorting in Rutherford's finest gay bar while thinking it is a hangout for other covert aliens. Meanwhile, the other Solomons plan a bank robbery, but fall prey to mutual suspicion.