I agree that there have been changes in how "dirty" themes and language have been treated in popular and folk song, but I would hesitate to think of it as an "evolution." It seems to me to be more of a cyclical thing. Certainly there was a a great amount of sexual repression during the Victorian era, but if you look at broadside ballads from the 17th and early 18th centuries, there was some stuff that would make a gangsta rapper blush. For instance I came across a fairly graphic broadside in the Pepys collection that was about a fifteen year old girls' desperate want of a dildo (with a "nonsense" refrain of "dildo-dildo-dildo dil"). Sexual topics are always at the forefront of human consciousness, and society during various eras deals with those topics in varying ways. Right now, in the U.S. it could be argued that we are in a new period of repression, but it won't last.