A Red Light District is a place where prostitution is common. The phrase was based in the Bible where
Rahab, a prostitute in
Jericho, aided
Joshua's spies with a red light.
Interestingly enough, the Japanese word "akasen" (red-line district) is of equal meaning to a red-light district.
Guy 1: Hey, have you ever visited
Kabukicho (Japan), Patpong (Thailand),
Wanchai (Hong Kong), Sunset
Boulevard (California {Hollywood}) and Venus Valley (Montana)? I sure did for the last couple of years, and all were awesome!
Guy 2: Well.. all of the cities you've visited.. they are all red-light districts. I'm sure you had a real "good" time there.