31 From this obedience that Father, who was not without a wife, was prepared to be without an only son, and that slain by himself. For I shall not without due cause call him an only son, concerning whom he heard the Lord say, “In Isaac shall there be called for you a seed.” Therefore how much sooner would he hear it, that he should be even without a wife, if this he were bidden? Wherefore it is not without reason that we often consider, that some of both sexes, containing from all sexual intercourse, are negligent in obeying precepts, after having with so great warmth caught at the not making use of things that are allowed.
Whence who doubts that we do not rightly compare unto the excellence of those holy fathers and mothers begetting sons, the men and women of our time, although free from all intercourse, yet in virtue of obedience inferior: even if there had been wanting to those men in habit of mind also, what is plain in the deed of the latter. Therefore let these follow the Lamb, boys singing the new song, as it is written in the Apocalypse, “who have not defiled themselves with women:” for no other reason than that they have continued virgins.
Nor let them on this account think themselves better than the first holy fathers, who used marriage, so to speak, after the fashion of marriage. Forsooth the use of it is such, as that, if in it there has taken place through carnal intercourse anything which exceeds necessity of begetting, although in a way that deserves pardon, there is pollution. For what does pardon expiate, if that advance cause no pollution whatever? From which pollution it were strange if boys following the Lamb were free, unless they continued virgins.
Source: On the Good of Marriage (New Advent)