Nice post. I don't really have comments about changing anything. I disagree with you about the whole "children and friends" part, but it's a point of view thing, and I think it's good you're sharing yours.
For the sake of discussion, though, my opinion and experience is that when you ask people "why...?" they get extremely defensive of it, and in turn aggressive. I get worse responses from "why did you want more than one child" than from "did you love your first child any less when the second was born?"
As for friends, people will say friends just happen, so it's not like they looked for them. I personally think that "how would you feel if one of your friends told you you can only have one friend at a time? What id they all told you that, and you had to pick one? Regardless of how much you love the one you pick, if you can pick at all, wouldn't you mourn the loss of all the other ones?" or something along these lines works better at making people think. Instead of questioning their choices by asking "why?", you present a hypothetical scenario and ask them to figure out how they would feel about it.
And I also don't understand your distinction between "you love all your children, right?" and "why did you want more than one child?" although personally I don't like phrasing it either way.
I think the way I like explaining it is such:
"There are many types of love. There is the love you have for your family, there is the love you have for your friends. And there is romantic love. For monogamous people, romantic love is the exception in that you only feel it for one person. For polyamorous people, romantic love is not an exception. Just like the others, you can and will feel it for several people, just like you love several members of your family or several friends at the same time."
As for the sexual aspect, people don't have trouble understanding that sex can occur without love and love without sex, so I fail to see why bringing love in the picture should make one think of sex. You can be polyamorous and asexual, you can be monoamorous and a swinger, you can have sex with your friends (that's what Friends With Benefits are all about), I think sex is relevant to a lot of people, but not necessarily relevant to a discussion about love. Many monogamous people would consider it cheating if you fell in love with someone else, regardless of the absolute lack of sex, and many people have an easier time with the idea of their partner having "sex on the side" than developing feelings for others.