NFP, if used continuously for “birth control” would be sin against the spirit for which it was intended. My understanding is that NFP may be practiced only in exceptional circuмstances with permission of a priest. It certainly should not be included in standard pre-marriage counseling as it is in the novus ordo and in some traditional circles. (If statistics are to be believed, the overwhelming majority of novus ordo Catholics ignore it and use regular birth control, which in a sense, is more honest!) If a couple is really in a situation where another baby or pregnancy is not doable or is dangerous, wouldn’t both agreeing to a period of celibacy be the better choice? Yes, each has conjugal rights, but sacrificing those rights for a time or even permanently may be what God calls for.