Sperm Banks and Moral Responsibility

When a man impregnates a woman accidentally through the usual means, he takes on responsibilities for the well-being of the child. The argument is that he acted in a way that he knew or should have known that this was a possibility and when it occurs, one has responsibilities for ones offspring. Why then is this not the case for sperm donors? They know that their action in making the donation will likely lead to a child, why do they not have any responsibility for its welfare? Is it that a contractual element exists? Can one really do away with such obligations contractually?