19 Therefore as many women as there are now, unto whom it is said, “if they contain not, let them be married,” are not to be compared to the holy women then, even when they married. Marriage itself indeed in all nations is for the same cause of begetting sons, and of what character soever these may be afterward, yet was marriage for this purpose instituted, that they may be born in due and honest order. But men, who contain not, as it were ascend unto marriage by a step of honesty: but they, who without doubt would contain, if the purpose of that time had allowed this, in a certain measure descended unto marriage by a step of piety.
And, on this account, although the marriages of both, so far as they are marriages, in that they are for the sake of begetting, are equally good, yet these men when married are not to be compared with those men as married. For these have, what is allowed them by the way of leave, on account of the honesty of marriage, although it pertain not to marriage; that is, the advance which goes beyond the necessity of begetting, which they had not. But neither can these, if haply there be now any found, who neither seek, nor desire, in marriage any thing, save that wherefore marriage was instituted, be made equal to those men.
For in these the very desire of sons is carnal, but in those it was spiritual, in that it was suited to the sacrament of that time. Forsooth now no one who is made perfect in piety seeks to have sons, save after a spiritual sense; but then it was the work of piety itself to beget sons even after a carnal sense: in that the begetting of that people was fraught with tidings of things to come, and pertained unto the prophetic dispensation.
Source: On the Good of Marriage (New Advent)