Uh, yeah it is.

That's the same reason why NPCs frown on fraternity little sisters.

The primary reason that little sister orgs were put to rest had to do with court challenges of the "single sex" status of fraternities and sororities. We have to be cautious to maintain certain requirements to ensure that we can continue to exist as single sex organizations and those groups violated the requirements.

I can see why the same would apply to not having official "brother fraternities" and that's why no NPC has an official relationship (excluding the historical ones) with one.