In the 17th-century broadside chosen by Child for his 'A' version of the ballad, the young woman - having been bewitched by the elf knight's 'magical horn' - is actually pretty keen to lose her maidenhead, at least until the moment he admits to being married with kids. It doesn't take too much imagination to read a sexual metaphor into her line: "I wish that horn were in my kist..." ('kist' = 'chest' or 'box').

I'm interested in Nerd's examples of the old riddles turning up in 1950s North Carolina, but even more interested to hear what actual answers the men would give to the conundrum. Would they really answer "penis" or "pubic hair"?

It may well be that the 'correct' answer to "What is deeper than the sea?" is "vagina", but when that riddle appears in ballads the commonest solution is "Hell". Are we to suppose that everyone knew the sexual reference but hid it it behind a biblical one?

Lastly, when Colleen Cleveland sings 'The Cambric Shirt' as learnt from her grandmother, she delivers the final lines:
"When you have done and finished your work
Then come to me, and I'll make your darn shirt"
with something approaching a sneer. The message seems to be more "Get stuffed" than "I'm saying No, but I mean Yes". That doesn't mean it never meant the latter, but it's quite possible that meanings can change according to context.