"That may well be the case at a subconscious level. But that doesn't go away because any or all of the people involved are in relationships."
Agreed. I'm lucky to work with men who don't seem to be looking for any extramarital dalliances, and our interactions reflect that. Being friendly and warm--what many would consider flirting--isn't required to affect the social status of those I interact with, but it can certainly help. And I think that's something that many people in a monogamous relationship learn to suppress, for various reasons.
You lost me. His point only applies to single people because people in monogamous relationships have learned to suppress "being friendly"? I have not seen anything that would suggest that is true, I could not guess the relationship status of anyone in my office based on their friendliness.