Pseudo-Chrysostom — as excerpted in the Catena Aurea on Mark 10:1-12

“(Vict. Ant. e Cat. in Marc.) The Lord calls by the name of adultery cohabitation with her who is not a man’s wife; she is not, however, a wife, whom a man has taken to him, after quitting his first; and for this reason he commits adultery upon her, that is, upon the second, whom he brings in. And the same thing is true in the case of the woman; wherefore it goes on, And if a woman shall put away her husband, and marry another, she committeth adultery; for she cannot be joined to another as her own husband, if she leave him who is really her own husband. The law indeed forbade what was plainly adultery; but the Saviour forbids this, which was neither plain, nor known to all, though it was contrary to nature.”