Apparently once upon a time there was an ointment one could rub on a broom - that was most popular amongst herbalists (such as many witches) - that was a hallucinogenic. One would ride the broom for masturbating purposes and the ointment would be absorbed through the mucus membrane of the vagina and give the rider a sensation of flying.

Now you will never look at Quidditch the same way again. 

http://kjerstifaret.tumblr.com/post/67019295839/a-comic-about-why-witches-are-stereotyped-as