William Smith, an online bookseller and blogger at Hang Fire Books, acquired a stack of old Sexology magazines. Sexology was one of publisher Hugo Gernsback's many titles (his most famous being Amazing Stories). Here's an illustration using obelisks to compare the difference between the "auto-erotic act" and the "normal marital act."
You can easily see–through the dramatic difference in "the height and girth of the obelisk[s]"–that the "gratification derived from the auto-erotic act is only about 60% as much as that of the normal marital act". But hey, I'm an obelisk half-full kind of guy.