I agree that this whole arrangement smells fishy.
I learned a long time ago that "fair" doesn't mean "equal." Different people have different needs and different capabilities.
For example, suppose one girlfriend gets really busy with work for a couple weeks and can't make time to see him. Is the other girlfriend not allowed to see SO until the one girlfriend has more free time? Who does that serve?
I can understand what he's going for, trying to make sure no one feels left out or discarded. But in real life, it's just not practical or even desirable to see every person for the exact same amount of time.
I guess it's his choice to make, if he wants to set it up that way and refuse to see certain people unless it works into his little plan. But that is someone I would definitely avoid.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker