70% seems like an extremely high number. Could it be that poly people with children are less likely to identify as poly (and use a different term for themselves)? Could it be that the definition of "parent" was lacking? (As Ari says, if it only include biological parents, it potentially excludes a bunch of people who might have raised the kids too)?
What SC said is true too, there would be less accidental pregnancies (I assume when you have several partners, you're more likely to be careful about that if you don't want children) and last I heard, 50% of pregnancies in the US were unplanned, which is a huge number.
But still, all of that doesn't add up to 70% when the CF population overall is more like 10% or something.
Another though, LGBT polys or polys who are sterile could be denied adoption due to their relationship configuration. It doesn't say that the non-parents don't wish they were parents.
So, it could be the stacking of a lot of things, including survey bias.