Check out this NYT article on parental rights of bio fathers in adoption cases. I was surprised to read that in many states if a man wishes to claim his parental rights he must have put his name on a "don't take my parental rights" registry. That seems a highly problematic system to me.
So unless a man registers whenever and wherever he has sex (assuming that's an option in his state), there's no way he can ensure that any child born from said sex act won't be given up for adoption against his wishes.
I'm curious to see Sen. Landrieu's proposed legislation which would create a a national registry and at least introduce some flexibility into the system.