10 Again, when explaining the witness of the apostle to the Galatians, “By the works of the law shall no flesh be justified,” I have spoken to the following effect: “Marriages also are works of the law. And for this reason there is a curse upon such as do not produce offspring. They are permitted, it is true, even under the Gospel; but it is one thing to concede an indulgence to what is a weakness and quite another to promise a reward to what is a virtue.” See my express declaration that marriage is allowed in the Gospel, yet that those who are married cannot receive the rewards of chastity so long as they render their due one to another.
If married men feel indignant at this statement, let them vent their anger not on me but on the Holy Scriptures; nay, more, upon all bishops, presbyters, and deacons, and the whole company of priests and levites, who know that they cannot offer sacrifices if they fulfil the obligations of marriage. Again, when I adduce evidence from the Apocalypse, is it not clear what view I take concerning virgins, widows, and wives? “These are they who sing a new song which no man can sing except he be a virgin.
These are 'the first fruits unto God and unto the Lamb,' and they are without spot. If virgins are the first fruits unto God, then widows and wives who live in continence must come after the first fruits— that is to say, in the second place and in the third.” We place widows, then, and wives in the second place and in the third, and for this we are charged by the frenzy of a heretic with condemning marriage altogether.
Source: Letters (New Advent)