XLVIII. The woman who has been abandoned by her husband, ought, in my judgment, to remain as she is. The Lord said, “If any one leave his wife, saving for the cause of fornication, he causes her to commit adultery;” thus, by calling her adulteress, He excludes her from intercourse with another man. For how can the man being guilty, as having caused adultery, and the woman, go without blame, when she is called adulteress by the Lord for having intercourse with another man?
XLIX. Suffering violation should not be a cause of condemnation. So the slave girl, if she has been forced by her own master, is free from blame.
L. There is no law as to trigamy: a third marriage is not contracted by law. We look upon such things as the defilements of the Church. But we do not subject them to public condemnation, as being better than unrestrained fornication.
Source: Letters (New Advent)