So even the great pleasure of sex is not sinful. An in case secundum rationem ordinatosounds like too great a stricture, Aquinas says that it can be reasonable to do something which inhibits reason, otherwise sleep would be a sin.
From Paul VI's encyclical, Humanae Vitae:“The sexual activity, in which husband and wife are intimately and chastely united with one another, through which human life is transmitted, is, as the recent Council [Vatican II] recalled, ‘noble and worthy.’”
No sin is "noble and worthy". Virtue is not sinful. But the conciliar statements of the Church, the most famous writing of her most famous theologian, and a Papal encyclical say that sex does not contradict virtue and is "noble and worthy". So the Catholic Church does not think sex is sinful.
One an say anything, but one must look at the actions.
Why MUST Mary remain a perpetual virgin?