89 If therefore the Church has not erred and does not err in teaching this, and consequently it is certain that the bond of marriage cannot be loosed even on account of the sin of adultery, it is evident that all the other weaker excuses that can be, and are usually brought forward, are of no value whatsoever. And the objections brought against the firmness of the marriage bond are easily answered. For, in certain circumstances, imperfect separation of the parties is allowed, the bond not being severed. This separation, which the Church herself permits, and expressly mentions in her Canon Law in those canons which deal with the separation of the parties as to marital relationship and co-habitation, removes all the alleged inconveniences and dangers. It will be for the sacred law and, to some extent, also the civil law, in so far as civil matters are affected, to lay down the grounds, the conditions, the method and precautions to be taken in a case of this kind in order to safeguard the education of the children and the well-being of the family, and to remove all those evils which threaten the married persons, the children and the State. Now all those arguments that are brought forward to prove the indissolubility of the marriage tie, arguments which have already been touched upon, can equally be applied to excluding not only the necessity of divorce, but even the power to grant it; while for all the advantages that can be put forward for the former, there can be adduced as many disadvantages and evils which are a formidable menace to the whole of human society.
Source: Casti Connubii (Vatican.va)