I think you confuse, "Low Fertility", with choosing to not have lots of children...

I don't believe there is a scientific study showing Atheists produce lower sperm counts, or any such indicator of fertility

Since when is having 1 child instead of 5 a bad thing?  I don't even see how this is a negative trait

If it were a race to have the most children... anyone with less than 20 children loses, right?