Anyone who chooses to have a child by using a sperm donation, is doing so because that is their wish. Sperm donors are helping these people to have children and should not be made to pay any child support to the parent(s). If this is the case, people will refuse to donate sperm, as they wouldn't want to have to pay child support.
Typically when a man donates sperm, he does it to receive payment for his DNA. He does not do it for a chance to pay child support. The care of the child should be taken solely upon the woman/family that sought the sperm in the first place. This defeats the purpose of sperm donation. If a woman wanted to receive support for her child, she would have to find a man to form a relationship with her.
The responsibility of a sperm donor should not include paying child support or any other future event for the child. Sperm donors would not want to participate if they knew that there would be future financial burdens for their role. It is important for donors to remain anonymous an distant from financial obligations.
Sperm donors should not be required to pay child support because that defeats the purpose of the arrangement. A lot of people use sperm donors simply because of the anonymity. Sperm donors are doing these patients a favor and in return the patients knowing accept full responsibility for the well being and care taking of the child.