I believe Barry has it right. People had a different attitude back in the old days. And the old shanty singers were raised in those days. I remember my grandfathers both telling stories that they broke off when a woman came into the room. Later those stories were not so bad as they seemed to think they were.

Still, they were largely young men, in a macho environment, competing with each other in the work and in the stories they would tell. Language, I've found, is usually pretty coarse in that kind of environment when it isn't tempered by the presence of women.