"If such a broad range of disabilities are permitted to attach to the status of pregnancy and motherhood, that status must be one of choice. And it is not sufficient to say that the women “chose” to have sexual intercourse, for she did not choose to become pregnant. As long as she is forced to bear such an extraordinarily disproportionate share of the pains and burdens of childrearing (including, of course, pregnancy and childbirth), then, to deprive her of the ultimate choice as to whether she will in fact bear those burdens violates the most basic aspects of “our American ideal of fairness” guaranteed and enshrined in the Fourteenth Amendment."