The response is as follows:- The first institution of marriage was effected in Paradise in such a way that there would have been "an unstained bed and honourable marriage" [Heb., xiii. 4] resulting in conception without ardor and birth without pain. The second, to eliminate unlawful movement, was effcted outside Paradise in such a way that the infirmity that is prone to foul ruin might be rescued by the uprightness of marriage. This is why the apostle, writing to the Corinthians says, "On account of fornication let each man have his own wife and each woman her own husband" [I Cor., vii. 2]. It is for this reason that the married owe a mutual debt to each other and cannot deny each other. So the apostle says, "Do not defraud one another except perhaps by consent for a time in order to give yourselves [more readily] to prayer. But return to it again lest Satan tempt you. [However I say this] on account of your incontinence" [I Cor., vii. 5]. Therefore, given that they are admonished to return to the natural use because of incontinence, it is clear that they are not commanded to join together solely for the procreation of  children. Yet marriage is not to be judged evil on that account, for what is done outside of the intention of generation is not an evil of marriage, but is forgiveable on account of the good of marriage which is threefold: Fidelity, Offspring, and Sacrament.

Huguccio gloss ad "sine ardore" ("Without ardor"):

            For, if man had not sinned, union would have been like the union of other bodily members and would have been without the fervor and itching of pleasure just like the union of other members is. For member would have been joined to member ... just like a slate to a slate."

Huguccio gloss ad "quod enim" ("Yet marriage"):

            The words are introduced by Master Augustine. But for better understanding, you should note with these words and the following points, that a man may know his wife for four reasons, that is for offspring, to pay the debt, for incontinence, or to satisfy lust and for the sake of pleasure. If for offspring, then coition is no sin, venial or mortal. Indeed, if done for love [caritas], it merits eternal life. The same is true when copulation is to pay the debt. Again, when it is for incontinence, coition is venial and the man sins venially. But when it is from lust or for the sake of pleasure, then the coition is a mortal sin and the man sins mortally. But whether the coition itself is a sin or not, it is never done without sin because it is always done and associated with some itching and pleasure. For in the emission of sperm there is always some fervor or pleasure which cannot be without blame. And these dicta assume that the man and his wife have sex according to the order of nature, for anyone who goes against nature always sins mortally and more seriously with his wife than with anyone else and should be punished more seriously... Note the difference between the two cases of husband-wife sex, for incontinence and for pleasure and lust ... In the
second case, he seeks to procure pleasure with hands or thought or passionate uses and incentives so he can do more than just have sex with his wife. Some however say that in both these cases coition is only a venial sin and man only sins venially whether he has his wife for incontinence or for satisfaction of lust. If therefore it seems to say elsewhere that he sins mortally, this is either said for the encouragement of continence and in detestation of the crime of adultery, or it is to be understood concerning coition against nature, or because he acts as an adulterer when he burns like an adulterer even with his own wife. But I do not waver from the words of the chapter ... and I say that the next chapter is to be understood as referring to incontinence and  that is properly to be understood concerning lustful enjoyment emerges later ...

Glossa Ordinaria ad "His ita respondetur" ("The response is as follows"):

            This is the second part of the Question, in which it says that marriage was instituted and permitted twice. First in Paradise simply for offspring. Secondly outside Paradise for offspring and to avoid the infirmity of the flesh, because marriage has a threefold good, Faith, Offspring and Sacrament. (John of Faenza)

            Casus: In this chapter Gratian distinguishes the institutions of marriage. One which was before sin and for the procreation of offspring. And the other which was given after sin for the avoidance of fornication. Which is proved by authority of the apostle saying: "On account of fornication", etc. So those who are joined in this way [marriage] cannot practise continence except by mutual consent. Which is proved similarly by authority of the apostle. So those who join ("coniuncti sunt") to avoid fornication ought to be called "coniuges". See the next chapter [C. 32. 2. 3] for proof.