Evidence suggests that it was common practice for various ancient civilizations to consider legally binding oaths made on genitalia.
In ancient Rome, testis meant both testicle and witness, leading to today's “testimony”. The endless pun possibilities were not lost on the ancients – go check out Plautus (in Latin, of course, how else can you enjoy the subtle word play? Wait, did I just say subtle referring to Plautus? Sorry, haven't had my coffee yet, I'm in a hurry this morning.)
There is a very extensive and well documented study by Joshua T. Katz on this issue, go search for it, if you have a passion for etymology.
In Athens, a man on trial would take his oaths in front of a table displaying the cut off pieces from a ram, a bull and a boar.
There is another much discussed example from the Genesis 24, where Abraham's servant takes an oath on his “thigh”; it's generally considered that “thigh” is an euphemism.
Since women did not hold any citizen rights in any of these civilizations, it makes sense that the legal oaths were designed exclusively for men, and what could be more exclusive than that? Boys will be boys, eh.