The rules are not arbitrary. They are very clearly defined. It's just that they're different for everybody, and for every different situation, and often depending on the mood of the person(s) involved, possibly influenced by (but not limited to) what they ate that day, how things are going with their significant other, recent setbacks at work, the weather, how their favorite sports team did yesterday, and/or whether or not they fucked up and left their cell phone at home that day. Also, there is the age of the person wearing said apparel, and of the person observing said wearing of said apparel, and possible relationships between them, physical, biological, or (in some cases) mechanical.
So yeah, there are a lot of factors in play. But it is far from arbitrary.