The comparison of genitalia is a ritual of masculinity characteristic of many species ; without going any further, all primates do it as part of the courtship. Penis size is associated with reproductive success, and they measure its size in an upright position.
In human beings, these games are restricted by cultural taboos. It is difficult for a man to see the member of another without dealing with a sexual situation. In certain cultures, public toilets allow that vision, and in our society, that function is exercised in the toilets. The moment of relief against the wall is the only moment when a male can have the naked sexual organs of another male of his species in view. And besides, they are all on their feet, like good primates.
