I went through an "is this really ok for me to want?" stage, for sure.
I heard the "you're being greedy" and "you just want to have your cake and eat it too" things.
What it comes down to is that if you can successfully maintain a loving committed relationship with two or more people and everybody be content, then you deserve that - it's not like it's easy. Because these people are poly they are not being "taken off the market" when it comes to monogamy - chances are they weren't ever ON the monogamous "market" anyway, so it's hardly being "greedy". And, because they are poly, they can still have other relationships.
"Listen, or your tongue will make you deaf." - Native American Proverb