102 This conformity of wedlock and moral conduct with the divine laws respective of marriage, without which its effective restoration cannot be brought about, supposes, however, that all can discern readily, with real certainty, and without any accompanying error, what those laws are. But everyone can see to how many fallacies an avenue would be opened up and how many errors would become mixed with the truth, if it were left solely to the light of reason of each to find it out, or if it were to be discovered by the private interpretation of the truth which is revealed. And if this is applicable to many other truths of the moral order, we must all the more pay attention to those things, which appertain to marriage where the inordinate desire for pleasure can attack frail human nature and easily deceive it and lead it astray; this is all the more true of the observance of the divine law, which demands sometimes hard and repeated sacrifices, for which, as experience points out, a weak man can find so many excuses for avoiding the fulfillment of the divine law.
Source: Casti Connubii (Vatican.va)