Marriage is the key
The Catholic faith teaches that sex is a gift from God even though that gift is frequently misused. Any reading of the Bible or even secular literature quickly shows how frequently and in how many ways men and women have misused the gift of their sexuality.
There is no direct biblical statement that sex is intended by the author of creation to be a renewal of the marriage covenant. However, we can arrive at that core statement by deduction. As will be shown in Chapter 17, “Biblical Foundations,” Sacred Scripture condemns adultery, fornication, homosexual behavior, contraception, masturbation, and bestiality. Thus the only form of sexual intercourse not condemned by Sacred Scripture is non-contraceptive intercourse between a man and woman who are married to each other. I will use the term “honest sex” or “honest sexual intercourse” to designate the sex act taught by Scripture and Tradition to be good: mutually voluntary, non-contraceptive intercourse by a validly married couple.
That leads to an obvious question: what is there about marriage that makes morally good the same physical act that is morally evil outside of marriage? Or to put it the other way, if honest sexual intercourse is (or can be) a moral good within marriage, why is it evil for those who are not married to each other? Certainly God knows that the degree of emotional love felt by unmarried persons is sometimes much stronger than that felt by many married couples. Let’s sharpen the focus a bit more. If Jim and Jane love each other, why is it the grave matter of mortal sin for them to have sexual intercourse on the day before they marry but morally good for them to celebrate their marriage with honest sexual intercourse after they have married?
John F. Kippley
Sex and the Marriage Covenant