It disappoints me a little bit, but it's the kind of things I would say, too. That or "if you hesitate between two people you love, go with neither, because if you were truly in love there would be no hesitation between two people, so obviously you don't actually love either".
It's the kind of things that are drilled into us from the get go, and with the majority of people actually working that way, I can understand why he would think and say that.
It's sad, but it's not unexpected. Only more knowledge and visibility of polyamory will change things like this.