Before I go, there was a quote (or a couple of quotes) from St. Thomas Aquinas that I thought I should share with everyone, I think it establishes his opinion with clarity:
“There are some who say that intercourse between married persons is not devoid of sin.
But this is heretical, for the Apostle says: “Let marriage be honorable in all and the bed undefiled.”[15]
Not only is it devoid of sin, but for those in the state of grace it is meritorious for eternal life. Sometimes, however, it may be a venial sin, sometimes a mortal sin. When it is had with the intention of bringing forth offspring, it is an act of virtue.
When it is had with the intent of rendering mutual comfort, it is an act of justice. When it is a cause of exciting lust, although within the limits of marriage, it is a venial sin; and when it goes beyond these limits, so as to intend intercourse with another if possible, it would be a mortal sin.” (
Catechism of St. Thomas Aquinas, pg. 80)
He says elsewhere:
“I answer that, Since no act proceeding from a deliberate will is indifferent, as stated in the Second Book (Sent. ii, D, 40, 1, 3; I-II, 18, 9),
the marriage act is always either sinful or meritorious in one who is in a 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 blessings, 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.” (
*Summa Theologica *Supplement Question 41)
I think these should put to end any speculation that St. Thomas believed that intercourse was only for procreation and not for union or that he believed that it was sinful.