In order to clear my mind of the borderline heresy Atila is expressing, I looked for a simple explanation of Acquinas and found this article by Father Basil Cole, OP. Some of this is over my head, but I can understand enough to see that Mr. Guimarães is not only wrong, but is deliberately misleading his followers by his misuse of St. Thomas.
Aquinas' Contribution to the Question of Chastity O.P. Marriage - Part ll https://www.osv.com/TheChurch/Practices/Article/TabId/665/ArtMID/13706/ArticleID/976/Aquinas-Contribution-to-the-Question-of-Chastity.aspx
It is when the reader becomes familiar with Thomas's teaching on marriage in the Supplementum, (a compilation of his earlier writings by his secretary, Blessed Reginald Piperno, perhaps other secretaries and perhaps other disciples,) that he can discover more about the range of the virtue of chastity.
How Is the Marriage Act Not Sinful?
. . . If we suppose the corporeal nature to be created by the good God, we cannot hold that those things which pertain to the preservation of the corporeal nature and to which nature inclines are universally evil; wherefore, since the inclination to beget an offspring whereby the specific nature is preserved is from nature, it is impossible to maintain that the act of begetting children is universally unlawful, so that it be impossible to find the mean of virtue therein; unless we suppose, as some are mad enough to assert, that corruptible things were created by an evil god... wherefore this is a most wicked heresy (ST, vol. 3). . .
Why Can the Marital Act be Meritorious?
When the Supplementum speaks about a marriage act being meritorious, it gives further light on the character of a chaste marital act. Before reading the reply to question 41a.4, the fifth objection is important to note:
Further, that which cannot be done without venial sin is never meritorious, for a man cannot both merit and demerit at the same time. Now there is always a venial sin in the marriage act, since even the first movement in such like pleasures is a venial sin. Therefore the aforesaid act cannot be meritorious.
The Supplementum begins its argument in the two sed contras: any act is meritorious if done from charity, and quotes Paul, saying ''let the Husband render the debt to his wife''; and since every act of virtue is meritorious when prompted by charity, rendering the debt is an act of justice. The reply gives further nuances:
Since no act proceeding from a deliberate will is indifferent..., the marriage act is always either sinful or meritorious in one who is in the state of grace. For if the motive for the marriage act be a virtue, whether of justice that they may render the debt, or of religion, that they may beget children for the worship of God, it is meritorious. But if the motive be lust, yet not excluding the marriage blessings, namely that he would by no means be willing to go to another woman, it is a venial sin, while if he exclude the marriage blessing, so as to be disposed to act in like manner with any woman, it is a mortal sin. And nature cannot move without being either directed by reason, and thus it will be an act of virtue, or not so directed, and then it will be an act of lust.
Someone who is simply inclined by the desire of pleasure alone and does not direct it to a good end, may be sinning venially, as long as his or her intention is not contrary to the goods of marriage. Any partner may be committing a mortal sin, however, if the act is done simply and exclusively from a love of pleasure as an end in itself as if the spouse were solely a ''pleasure machine.''
The first movements of sexual arousal mentioned in the fifth objection can be the occasion of virtue or vice depending on what a person does with these movements as is the case with any temptation against any virtue.
Of themselves, the first movements of any sin are called in many places in the Summa ''venial sins,'' but analogously so, not univocally because they do not yet engage the consent of the will but are in some sense disorderly due to original sin and frequently the residue of past personal sin. So they can be turned to virtue or sin by the intellect and will.
The Goods of Marriage as Motivators
In question 49, article 4, the Supplementum explains more what they mean by the goods of marriage. It begins with a very pertinent objection, which is the first one:
It would seem that the marriage act cannot be altogether entirely without sin by the aforesaid goods of marriage [Here understood as offspring, fidelity and sacrament]. For whoever allows himself to lose a greater good for the sake of a lesser good sins because he allows it inordinately. Now the good of reason which is prejudiced in the marriage act is greater than the three marriage goods. Therefore the aforesaid goods do not suffice to excuse marital intercourse.
Objection three offers another hurdle to overcome in order to appreciate marital chastity:
Further, wherever there is immoderate passion there is moral vice. Now the marriage goods cannot prevent the pleasure in that act from being immoderate. Therefore they cannot excuse it from being a sin.
The reply is a lesson in fundamental moral theology, the second half of which shows what a signified act is:
...Now a human act is said to be good in two ways. In one way by goodness of virtue, and thus an act derives its goodness from those things which place it in the mean. This is what ''fidelity'' and ''offspring'' do in the marriage act... In another way, by goodness of the ''sacrament,'' in which way an act is said to be not only good, but also holy, and the marriage act derives this goodness from indissolubility of the union, in respect of which it signifies union of Christ with the Church. Thus it is clear that the aforesaid goods suffice to render the marriage act innocent.
Father Cole, O.P., is associate professor at the Dominican House of Studies in Washington, D.C., and is the author of The Hidden Enemies of the Priesthood