17 That marriage can take place of persons first ill joined, an honest decree following after, is manifest. But a marriage once for all entered upon in the City of our God, where, even from the first union of the two, the man and the woman, marriage bears a certain sacramental character, can no way be dissolved but by the death of one of them. For the bond of marriage remains, although a family, for the sake of which it was entered upon, do not follow through manifest barrenness; so that, when now married persons know that they shall not have children, yet it is not lawful for them to separate even for the very sake of children, and to join themselves unto others.
And if they shall so do, they commit adultery with those unto whom they join themselves, but themselves remain husbands and wives. Clearly with the good will of the wife to take another woman, that from her may be born sons common to both, by the sexual intercourse and seed of the one, but by the right and power of the other, was lawful among the ancient fathers: whether it be lawful now also, I would not hastily pronounce. For there is not now necessity of begetting children, as there then was, when, even when wives bare children, it was allowed, in order to a more numerous posterity, to marry other wives in addition, which now is certainly not lawful.
For the difference that separates times causes the due season to have so great force unto the justice and doing or not doing any thing, that now a man does better, if he marry not even one wife, unless he be unable to contain. But then they married even several without any blame, even those who could much more easily contain, were it not that piety at that time had another demand upon them. For, as the wise and just man, who now desires to be dissolved and to be with Christ, and takes more pleasure in this, the best, now not from desire of living here, but from duty of being useful, takes food that he may remain in the flesh, which is necessary for the sake of others; so to have intercourse with females in right of marriage, was to holy men at that time a matter of duty not of lust.
Source: On the Good of Marriage (New Advent)