The idea that a person cannot be mono if they are with a poly is not that uncommon. In our local poly community there are people who consider me poly for this reason. It comes down to what a person considers monogamous. Some people see monogamy to be a function of relationship dynamic..1 with 1, where others see it as an internal orientation similar to sexual preference...1 only loves 1. There is a big difference between the two. I am going through a transitional phase of what it means to be monogamous myself.
Don't beat up on him too bad for this idea...there's no harm in it unless he misses out on valuable experiences of people in his situation.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over