I think committing to a poly relationship is the same as committing to a mono relationship in this respect. If you are truly committed, there is no primary and secondary. Honestly, could any of you live as a "secondary" in your partners' lives? Could you live with people you love, commit your life to them, knowing that you won't ever be as important to them as they are to you? I don't believe that's a commited relationship at all, and wouldn't want to have kids if that was the case. It's easy to see things a certain way when you already have "the one" and are looking for the icing on your cake. But what if you start single, and find "the ones?" That doesn't mean you want to be secondary to each of them forever. There comes a point where a committment is made, and it must be complete, or it might as well be non-existent. I think to limit something like childbearing to one relationship says "this other relationship is more important to me than you will ever be." And for me, that's not acceptable.
What defines important? Commitment? Is it worth it to be 'the icing'? Is it worth it to have any sort of relationship? Can one ever really depend on anything or anyone not under their direct and complete control? More crap to think about.