The Scarborough Fair song that did the rounds after it was included in 'The Graduate' - (Simon and Garfunkle) is derived from Child ballad 2 - 'The Elfin Knight' - not so much 'sexual' as a battle of wits between a man (often a stranger' and woman
The 'Parsley, sage.... plants are good luck charms rather than sexual references.
Usually the prize is marriage (if she can answer his riddles)
In the case of 'Riddles', the competition is one where the woman proves her equality by giving as good as she gets with impossible questions
In the case of 'The False Knight on the Road' (Child 3), the competition is between a schoolboy and a stranger he meets on the road, who turns out to be the Devil - the boy, in the course of the competition, exposes him for who he is and saves his own sould by doing so.
The one to have lasted the longest in the tradition in Ireland is 'Captain Wedderburn's Courtship' (Child 46)
"Song of Lugnasadh"?
Never come across this one - possibly a modern reference.
Lughnasa (the feast of the God Lugh) marks the beginning of Harvest in Ireland
Jim Carroll