Nope - I was the one who used the "jealousy" analogy. Your post was more akin to "they have bad things to say about it because they have a secret desire to BE it" and my brain immediately flipped to "they say those things because they're jealous of you". It felt the same way - well meaning, but not right (in my case).
The problem with both sentiments (IMO) is that it sets up a premise that makes it easy to avoid looking at the real reasons people have a hard time with an idea. They hate poly? They must want a second relationship! Aha! Wouldn't THAT be ironic!
Well... no. It's not usually that simple. There are a myriad of reasons why people would be against such a thing. If (in my friends' case) they see something they feel is hurting their friends, then Poly is obviously a "BAD THING". If they don't like it for religious reasons, that's another completely different case. There could be other reasons. Each reason, if you're going to discuss it with people, needs a different argument against it. Is it abusive? Well, no, not in my case, because of A,B,C. Is it against your religion, well okay, but your religion is not mine. Is it a guy who can't keep his dick in his pants? Well, in our relationship, it's more x,y,z and not that at all... or, who cares if he likes variety in his sex life, if everyone else is okay with it?
Reducing the argument to, "Well, they just want that for themselves," doesn't allow us to get to any of those reasons, and you really can't have a good discussion with people when you come at them with this argument. "Oh, you just want to be me," doesn't get taken well, and doesn't show the other person that you respect them and their opinion (which goes miles in trying to get them to respect you and yours).
Remember, I'm mono. Being told that any of the problems I had with poly in the beginning were because I just want a second partner of my own is laughable. I don't want that. Truly. And if anyone ever said that to me, depending on who it is, I may just shut the argument off right there, because it's obvious they don't want to hear the real reason behind any of it. May as well move on to talking about the weather. Or sports.
Hey, how about them Mets?