First time I ever heard "the sex verse" was from the Dransfields way back when. I feel it's pretty rare but it makes sense that it be there, of course, since the "father of a son" line is common enough. Likely they got it from Penguin...

They sing it very close to the DT verse:

And so early in the morning at the dawning of the day *
They went out into the hayfield to have some sport and play;
And what they did there, she never would declare
But she never more complained of his growing.

*additional verse from Penguin Book of English Folk Songs, Williams and Lloyd

Any notion how common/old/widespread the verse actually is?