Augustine of Hippo, De Bono Coniugali, ch. 16; later quoted by Thomas Aquinas, Summa Theologica, part 2, question 153, article 2, adding: "Wherefore just as the use of food can be without sin, if it be taken in due manner and order, as required for the welfare of the body, so also the use of venereal acts can be without sin, provided they be performed in due manner and order, in keeping with the end of human procreation."https://en.wikiquote.org/wiki/Human_sexual_activity