The point was that prohibition does  harm, in pursuit of its end. There is a moral obligation to consider that harm, before embracing prohibition as a means. If one  considers the harm, and finds that it is of a serious nature, and that there would be no clear way to redress the harm after the fact, then one might find that prohibition was not an appropriate means to the end of reducing abortion. 

The primary consequence of prohibition would be to enshrine in law the premise that women do not have a right to choose not to bear a pregnancy to term and give birth. Women's human capacities of conscience and choice in the disposition of their bodies, would be stripped from them in the event of pregnancy, rendering them slaves, or beasts, with respect to giving birth. 

One may be pro-life, without endorsing the perverse cruelty of  using pregnancy as the excuse for diminishing women's own personhood. To be coherently and thoughtfully pro life might actually prohibit endorsing such means.