This seems like many other agreements two people might make, and not in a class by itself. On the whole, we should keep our promises, but we don't want the law to step in and enforce all of them. If a woman promises not to have an abortion, then she surely she should keep the promise. It would be the same in the other direction. A woman might ask a man to agree not ask her to have an abortion, if she should become pregnant. Morally, both have at least a prima facie obligation to keep their promises.

The law is another matter. If by making any promise we incurred the risk of being dragged into a courtroom, we'd make promises very sparingly. And then a valuable way of managing our personal relationships would be virtually lost. In this particular situation, either party would be able to foresee that the other could renege on the agreement. After all, as we all know, it's hard to predict what one will really want to do about something as life-changing as a pregnancy. A reasonable person wouldn't trust the other's word completely, and would avail themselves of good contraception.