Religious people see "be fruitful and multiply" as a literal command from G-d and one of the fundamental points of religion. So while religious culture can evolve, the evolution of this attitude isn't a flexible point.
On the flip side, secular culture has no intrinsic reason for "family values" - which is why, I think, atheist culture over time devolves to childlessness - because reasons "why not" are more immediate and in your face, vs "why yes."
So yes it's "culture" but what the culture is is obviously determined by your underlying beliefs and that which you consider eternal and that which you consider negotiable.