I have heard of this concept before - "I won't tell them about poly right away because that will just scare them away." The implication being that somehow this person will see what a great person you really are, and, when you mention poly, won't be comfortable with it, but will be willing to work with you because you are such a great person they don't want to lose you.... (that was truly the reasoning I was given by someone who put "single" on his OKCupid profile, without any mention of polyamory in his descriptions...
My take on it is that if this person is going to run away when you mention poly, then you never really had them truly interested in you in the first place - instead they were interested in the person they thought you might be. So in other words, you didn't "lose them" because you never really 'had them" in the first place.
If poly really is as natural as breathing to us, then it should come up naturally in the conversation, as far as i am concerned. I have two people who I share my life with, to whom I am committed, and love very much. if I were meeting someone that I thought was a prospective date, then I would have a hard time not mentioning them. So hard, that it would feel to me like I was lying.
I know that in the early days of dating we tend to be on our best behaviours - we want the prospective dates to see our best side. By not showing your poly side you are acting as though it is something to be ashamed of and hidden until the person has got to know you a bit better. Why is that?
"Listen, or your tongue will make you deaf." - Native American Proverb