I think they mean using a characteristic that some people gave as an insult to someone else. It’s unfair to mock someone else by using a characteristic that someone else has, which they cannot control (in this case, an aspect of their body shape, like a micro penis). The same can be said of insulting someone by calling them “retarded”, for example. It’s always demeaning to the person who has that characteristic, when it is used as an insult toward another person.