A comic about why witches are stereotyped as riding broom:
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.
I had read that it was an euphemism, the broom was a metaphor for the Devil’s dick, but this is just as cool.