They focus a lot on Psalm 127:3-5 which says,
3 Sons are a heritage from the LORD,Biblegateway Link. I provide the Biblegateway link in case you want to read the whole Psalm, or those around it.
children a reward from him.
4 Like arrows in the hands of a warrior
are sons born in one's youth.
5 Blessed is the man
whose quiver is full of them.
They will not be put to shame
when they contend with their enemies in the gate.
I think these folks have nailed the intention of the Psalmist. Furthermore, the idea of having lots of children to make your nation strong is throughout the Bible. So, while this scripture may give them a good slogan, there is really much support for the idea.
Here's where I think they are wrong. First, this was the conventional wisdom of the day. It is not unique to find this in the bible. Unlike scriptures calling for mercy and restraint in a vicious time, see e.g. taking only a eye if someone takes your eye, or scriptures calling for kindness to the poor, scriptures encouraging big families are not a product of the unique something that I believe keeps the Bible relevant. Second, having large families is destructive to our planet and not at all helpful to our nation. Educated children, that makes our nation strong, not lots of children. So, I think they are wrong because they are focusing on an unimportant accidental message contained in the Bible.
It is wrong to say these people are misreading the Bible. The Bible encourages big families, patriarchal relationships in which the wife or wives are obedient to their husband, and unity of religion and government. But those reflections of the culture in which it was written are just not important. Am I cherry picking? Sure. But it isn't that I'm just taking the things that already conform to my values (I hope) I'm taking the more profound spiritual lessons.
UPDATE: Re-reading my post I came across to strong in opposition to big families. I think if people are fulfilled by big families, if they love and enjoy them, big families are great. I feel this exactly as strongly as I feel that it is a blessing that couples can feel free today not to have any children. I should have said, that a policy to encourage larger families than people would have otherwise is destructive to our planet and does not help make our nation strong.