Sex in McMurtry's "Lonesome Dove"

One of Larry McMurtry's more charming and clever turns of phrase has to do with the many colorful ways he was able to describe the male sex organ and the act of sexual intercourse. I wonder if he just came up with these phrases on his own or if this really was a kind of 19th Century slang that he uncovered in his research. Either way, it was one of the more amusing elements of the narration.