"The legal reasoning is that child support is a right of the child rather than the parent, and as such any contract by the parent waiving it is invalid. I can't speak for Sweden, but US courts have upheld the principle on numerous occassions."
Yes, it's the right of the child. That's why the non-related lesbian should have to pay child support. I was under a different impression, though admittedly a very vague one, about the rights of sperm donors in the U.S. I mean, I realize it's not good to have a fatherless household, but forcing this guy to pay won't change that; all it will do is screw over this guy who made clear from the very beginning what he was doing. He might be a liberal douche, but that doesn't change the financial responsibility of the lesbian who was with the mother at the time.