Your "biological fact" is simply lawyerly opinion. You argue for "case by case" studies when it suits you but insist that we all agree with your opinionated sweeping generalizations. Screw that. It allows you to be the arbitor of what "important" means and to change that meaning whenever it suits your litigation needs. To say that you still think fathers are "important" in one hand while dismissing them out of the other hand is either sloppy argument technique or more of your blind anti-man prejudice.
As I told another poster: If the courts - and I'm talking about you, personally, here - really believed that fathers were important in any way, shape, or form, they would adjudicate and enforce custody agreements with as vigorously as they enforce support payment agreements. That men who actively voice this same opinion get called "whiners" and "deadbeats" as often by lawyers as by their feminist cheerlaeading section tells me all I need to know.
I notice how you refuse to understand that children don't stay young, so a mother is not all they need. It's the children that grow up without dads that keep you lawyer types busy. You don't see like much of a lawyer based on the argument you made here.