34 But those of ours who have wives we advise, with all our power, that they dare not to judge of those holy fathers after their own weakness, comparing, as the Apostle says, themselves with themselves; and therefore, not understanding how great strength the soul has, doing service unto righteousness against lusts, that it acquiesce not in carnal motions of this sort, or suffer them to glide on or advance unto sexual intercourse beyond the necessity of begetting children, so far as the order of nature, so far as the use of custom, so far as the decrees of laws prescribe.
Forsooth it is on this account that men have this suspicion concerning those fathers, in that they themselves have either chosen marriage through incontinence, or use their wives with intemperance. But however let such as are continent, either men, who, on the death of their wives, or, women, who, on the death of their husbands, or both, who, with mutual consent, have vowed continence unto God, know that to them indeed there is due a greater recompense than marriage chastity demands; but, (as regards) the marriages of the holy Fathers, who were joined after the manner of prophecy, who neither in sexual intercourse sought anything save children, nor in children themselves anything save what should set forward Christ coming hereafter in the flesh, not only let them not despise them in comparison of their own purpose, but let them without any doubting prefer them even to their own purpose.
Source: On the Good of Marriage (New Advent)