child support

Two women wanted a child. So they went to a sperm bank and purchased the sperm. Then subequently the relationship broke up and the child was to be raised in a single parent family. Not too bad these days you might say.
But the parent decided to ask for more money and the poor doner was hit with child support payments. He gave of his own free will yet was tracked down and made to pay for a child he didn't know or want any relationship with.
Where is the common sense in that or indeed the justice?