16 Therefore if haply, (which whether it can take place, I know not; and rather think it cannot take place; but yet, if haply), having taken unto himself a concubine for a time, a man shall have sought sons only from this same intercourse; neither thus is that union to be preferred to the marriage even of those women, who do this, that is matter of pardon. For we must consider what belongs to marriage, not what belongs to such women as marry and use marriage with less moderation than they ought.
For neither if each one so use lands entered upon unjustly and wrongly, as out of their fruits to give large alms, does he therefore justify rapine: nor if another brood over, through avarice, an estate to which he has succeeded, or which he has justly gained, are we on this account to blame the rule of civil law, whereby he is made a lawful owner. Nor will the wrongfulness of a tyrannical rebellion deserve praise, if the tyrant treat his subjects with royal clemency: nor will the order of royal power deserve blame, if a king rage with tyrannical cruelty.
For it is one thing to wish to use well unjust power, and it is another thing to use unjustly just power. Thus neither do concubines taken for a time, if they be such in order to sons, make their concubinage lawful; nor do married women, if they live wantonly with their husbands, attach any charge to the order of marriage.
Source: On the Good of Marriage (New Advent)