When you hear song lyrics how often do you know what they mean, I am still not sure I know what Fergie is talking about with a "London Bridge"?

So as I was driving into work this morning I was listening to a station which does a request jam on Fridays. One of the song’s in there was Fergie’s London Bridge. I started thinking about how many times I have heard that song and figured that the phrase was a euphamism. But today I was asking myself what does it really mean? Did some of you jellies know what this means without having to do an internet search? Even after having attempted to do some internet searches I am still not sure I know. How many of you have a story like this about music you can share?