The three fonts of morality are the basis for determining if any act is moral or immoral:
- intention – you do not intend the miscarriages, but instead you intend the goods of marriage, including children.
- moral object – miscarriages are not abortions, so there is no question of an intrinsically evil act. And marriage certainly has good moral objects.
- consequences – this is the font that will determine whether or not it is moral for you to marry. The reasonably anticipated good consequences must outweigh the reasonably anticipated bad consequences. The goods of marriage are many and weighty. However the foreseen but unintended death(s) of prenatals is also of substantial weight. NFP can certainly lessen the risk of pregnancy with miscarriage, so this good also carries some moral weight.
You will have to weigh these goods yourself, and perhaps seek some advice from a priest. It would also be helpful for whomever may assist you in this decision to know what the likelihood is of a successful pregnancy, versus the likelihood of a miscarriage.