11 And yet not to these themselves is marriage a sin; which, if it were chosen in comparison of fornication, would be a less sin than fornication, and yet would be a sin. But now what shall we say against the most plain speech of the Apostle, saying, “Let her do what she will; she sins not, if she be married;” and, “If you shall have taken a wife, you have not sinned: and, if a virgin shall have been married, she sins not.” Hence surely it is not lawful now to doubt that marriage is no sin.
Therefore the Apostle allows not marriage as matter “of pardon:” for who can doubt that it is extremely absurd to say, that they have not sinned, unto whom “pardon” is granted. But he allows, as matter of “pardon,” that sexual intercourse, which takes place through incontinence, not alone for the begetting of children, and, at times, not at all for the begetting of children; and it is not that marriage forces this to take place, but that it procures pardon for it; provided however it be not so in excess as to hinder what ought to be set aside as seasons of prayer, nor be changed into that use which is against nature, on which the Apostle could not be silent, when speaking of the excessive corruptions of unclean and impious men.
For necessary sexual intercourse for begetting is free from blame, and itself is alone worthy of marriage. But that which goes beyond this necessity, no longer follows reason, but lust. And yet it pertains to the character of marriage, not to exact this, but to yield it to the partner, lest by fornication the other sin damnably. But, if both are set under such lust, they do what is plainly not matter of marriage. However, if in their intercourse they love what is honest more than what is dishonest, that is, what is matter of marriage more than what is not matter of marriage, this is allowed to them on the authority of the Apostle as matter of pardon: and for this fault, they have in their marriage, not what sets them on to commit it, but what entreats pardon for it, if they turn not away from them the mercy of God, either by not abstaining on certain days, that they may be free to pray, and through this abstinence, as through fasting, may commend their prayers; or by changing the natural use into that which is against nature, which is more damnable when it is done in the case of husband or wife.
Source: On the Good of Marriage (New Advent)