Sexual morality is a programme for helping people to distinguish true love from its counterfeits. It keeps love from being wasted. An old Irish phrase for sexual sin described it as 'making a spoiling of love'. In this respect sexual morality is not different from the rest of Christian morality. Moral laws are all of them statements of what is, in this or that situation, the truly loving thing to do, for others and for oneself.
Christian teaching says no only to what is unloving.
It says yes to love provided the love is truthful and honest. Wrong uses of sex are wrong because they are never the truly loving thing to do.