I'm not sure this particular line was discussed in the following thread:
http://www.mudcat.org/thread.CFM?threadID=9589
This is one line on the subject I found in Bert Lloyd:
[1967:] 'Lay the bent to the bonny broom' (a phrase of 'physiological significance' - 'bent' = 'horn' - says Miss Margaret Dean-Smith who has a sharp sense for euphemism). (Lloyd, England 153f)Could 'broom' be of equally physiological significance? - Susanne