Because it was a joke - a joke that caught on and spread - like folk music spreads and changes in the process. One of the funniest and most ironic versions is by Fats Waller, who managed to take the piss out of tunes yet, at the same time, invest them with great musicality. Have a listen to his version - after which no-one could take the song seriously. It's a bit of 1920s rubbish - but great fun to play.

And it's not really a sexual element - it's just a silly phrase which actually parodies the song.