I disagree.
The real reasons you take issue with it:
1. Performance anxiety - you don't want to have to think that they may be watching you and thinking that your dicks appear much smaller in real life and without the aid of wide angle lens distortion working in your favor.
2. Control - you don't want the models having anyone to give them support when you are asking them to do shit that they feel uncomfortable about.
3. Subterfuge - you want to be able to things without having to worry about witnesses, second opinions, critiques, etc.
I have shot enough and worked for enough people - including Max Hardcore - who inspired Danza, Steve Sweet, etc., to know what the dealio is.
If a model is more comfortable or has a partner that gets off on watching then more power to them. If the so-called "spectator" starts interfering with production then kick them both off set. Trust me, I have had my share of problems with boyfriends too... ask Mike South about the time in Vegas when one ran out of the room with my camera. But there is nothing illegal about having a friend, spouse, bodyguard whatever on set, nor is it unprofessional until that person(s) behaves unprofessionally.
You may be shooting porn, and you are right that it should be done with professionalism, but it should also have an element of courtesy and respect for the models so that they feel inspired to do a good "job".
