Matthew 19:8

Moses, because of the hardness of your hearts, permitted you, &c. Whether this was permitted in the old law, so that the man who was divorced from his wife could marry another woman, is disputed. Some think this second marriage was still unlawful for the man or woman so separated to marry another. (Witham) --- The latter part of this verse, of St. Paul, (Romans vii. 3,) and the constant tradition of the Church, shew that the exception only refers to separation, but not to the marrying another during the life of the parties. In this place Christ restores the original condition of the marriage state, and henceforth will have it to be a perfect figure of the hypostatic union of his divine person with our human nature, as also of his nuptial union with his Church, and consequently that it should be indissoluble. (Tirinus)