13 What therefore he says, “She, that is unmarried, thinks of the things of the Lord, that she may be holy both in body and spirit;” we are not to take in such sense, as to think that a chaste Christian wife is not holy in body. Forsooth unto all the faithful it was said, “Do you not know that your bodies are a temple of the Holy Ghost within you, Whom you have from God?” Therefore the bodies also of the married are holy, so long as they keep faith to one another and to God.
And that this sanctity of either of them, even an unbelieving partner does not stand in the way of, but rather that the sanctity of the wife profits the unbelieving husband, and the sanctity of the husband profits the unbelieving wife, the same Apostle is witness, saying, “For the unbelieving husband is sanctified in the wife, and the unbelieving wife is sanctified in a brother.” Wherefore that was said according to the greater sanctity of the unmarried than of the married, unto which there is also due a greater reward, according as, the one being a good, the other is a greater good: inasmuch as also she has this thought only, how to please the Lord.
For it is not that a female who believes, keeping married chastity, thinks not how to please the Lord; but assuredly less so, in that she thinks of the things of the world, how to please her husband. For this is what he would say of them, that they may, in a certain way, find themselves obliged by marriage to think of the things of the world, how to please their husbands.
Source: On the Good of Marriage (New Advent)