Explanation: Romans, unlike some Greek sources, did not find a small penis to be attractive - but an overly large penis was considered comical and barbaric. I suppose the "I'm perfectly average!" was the real mating call of the Roman man.
Also, going down on a woman was considered emasculating. Nonetheless; it was acknowledged as often happening (and used as an insult), and male prostitutes are recorded advertising their prices for cunnilingus.