I think you'll find its not who is on the flight deck but about the number of times the locked flight deck door is opened. It doesn't matter if its your wife child or mistess, on a several hour leg they are going to be in and out of the flight deck. Restricting the number on the flight deck is in keeping with the locked door policy.
I don't agree with it as much as anyone else but a secure FD door opening and closing unnecessarily is seen as a security risk.