I would tend to agree with two points raised:
1. Poly parents barely have time to brush their teeth, let alone fill out surveys.
2. Poly self-identified people tend to make intentional choices, and may be less likely to become "parents by accident."
Absolutely any segment of the population with a higher tendency to make careful decisions and take thought-out actions to follow those decisions is going to have a lower percentage of parents, because they'll have a small fraction of members who get pregnant carelessly.
I'm pretty sure that when Mr. Harper's new "voluntary long form" census results are in, we'll see a lot fewer parents, professionals, and students filling them out. Those people just don't have gobs of spare time for voluntarily filling out surveys if it's not legally required. It will suddenly look like Canada is populated by statisticians, accountants, and actuaries, i.e. people who love filling out forms just for fun.
I don't need labels to define me. They're sticky and I hate the glue they leave behind.