Contracts to waive child support are not enforeable, because the obligation is nominally to the child not the custodial parent.
And the custodial parent doesn’t always have discretion either. Anyone applying for social assistance for a child is required to name a father, and from that point the welfare people will go after him on their own. (I imagine most of these women are not applying for social assistance, but circumstances change.)
Sperm banks are specifically exempted by law.