Objection 1. It would seem that the marriage act can be excused even without the marriage goods. For he who is moved by nature alone to the marriage act, apparently does not intend any of the marriage goods, since the marriage goods pertain to grace or virtue. Yet when a person is moved to the aforesaid act by the natural appetite alone, seemingly he commits no sin, for nothing natural is an evil, since "evil is contrary to nature and order," as Dionysius says (Div. Nom. iv). Therefore the marriage act can be excused even without the marriage goods.
Objection 2. Further, he who has intercourse with his wife in order to avoid fornication, does not seemingly intend any of the marriage goods. Yet he does not sin apparently, because marriage was granted to human weakness for the very purpose of avoiding fornication (1 Corinthians 7:2-6). Therefore the marriage act can be excused even without the marriage goods.
Objection 3. Further, he who uses as he will that which is his own does not act against justice, and thus seemingly does not sin. Now marriage makes the wife the husband's own, and "vice versa." Therefore, if they use one another at will through the instigation of lust, it would seem that it is no sin; and thus the same conclusion follows.
Objection 4. Further, that which is good generically does not become evil unless it be done with an evil intention. Now the marriage act whereby a husband knows his wife is generically good. Therefore it cannot be evil unless it be done with an evil intention. Now it can be done with a good intention, even without intending any marriage good, for instance by intending to keep or acquire bodily health. Therefore it seems that this act can be excused even without the marriage goods.
On the contrary, If the cause be removed the effect is removed. Now the marriage goods are the cause of rectitude in the marriage act. Therefore the marriage act cannot be excused without them.
Further, the aforesaid act does not differ from the act of fornication except in the aforesaid goods. But the act of fornication is always evil. Therefore the marriage act also will always be evil unless it be excused by the aforesaid goods.
I answer that, Just as the marriage goods, in so far as they consist in a habit, make a marriage honest and holy, so too, in so far as they are in the actual intention, they make the marriage act honest, as regards those two marriage goods which relate to the marriage act. Hence when married persons come together for the purpose of begetting children, or of paying the debt to one another (which pertains to "faith") they are wholly excused from sin. But the third good does not relate to the use of marriage, but to its excuse, as stated above (Article 3); wherefore it makes marriage itself honest, but not its act, as though its act were wholly excused from sin, through being done on account of some signification. Consequently there are only two ways in which married persons can come together without any sin at all, namely in order to have offspring, and in order to pay the debt. otherwise it is always at least a venial sin.
Reply to Objection 1. The offspring considered as a marriage good includes something besides the offspring as a good intended by nature. For nature intends offspring as safeguarding the good of the species, whereas the offspring as a good of the sacrament of marriage includes besides this the directing of the child to God. Wherefore the intention of nature which intends the offspring must needs be referred either actually or habitually to the intention of having an offspring, as a good of the sacrament: otherwise the intention would go no further than a creature; and this is always a sin. Consequently whenever nature alone moves a person to the marriage act, he is not wholly excused from sin, except in so far as the movement of nature is further directed actually or habitually to the offspring as a good of the sacrament. Nor does it follow that the instigation of nature is evil, but that it is imperfect unless it be further directed to some marriage good.
Reply to Objection 2. If a man intends by the marriage act to prevent fornication in his wife, it is no sin, because this is a kind of payment of the debt that comes under the good of "faith." But if he intends to avoid fornication in himself, then there is a certain superfluity, and accordingly there is a venial sin, nor was the sacrament instituted for that purpose, except by indulgence, which regards venial sins.
Reply to Objection 3. One due circumstance does not suffice to make a good act, and consequently it does not follow that, no matter how one use one's own property, the use is good, but when one uses it as one ought according to all the circumstances.
Reply to Objection 4. Although it is not evil in itself to intend to keep oneself in good health, this intention becomes evil, if one intend health by means of something that is not naturally ordained for that purpose; for instance if one sought only bodily health by the sacrament of baptism, and the same applies to the marriage act in the question at issue.
Source: Summa Theologica (New Advent)