If human beings were simply living bodies ("ensouled bodies", because a soul is an animating form) like other animals, there would be no very strong reason against promiscuity. Evolutionary and social reasons would not suffice. Psychological factors might well be against it. Not everyone is inclined, like
swans or elephants, towards monogamy. Nor would it be easy to explain why two people who felt themselves to be deeply in love should not do what nature presses them to do, and express that love outside the sacrament.
Yet man is not just an ensouled body; he is also spirit. That is to say, there is in the human soul an interior dimension, an inner chamber, in which our particular likeness to God consists, and where our true freedom resides. This fact, which much of modern thought conspires to deny or obfuscate, transforms utterly our relationship to one another. We cannot, save by a denial of the most important part of our humanity, act towards each other as other animals do, or as our unspiritual nature on its own inclines us.
In human beings, acts tending to reproduction can and should be something free and personal, acts of love. So far, the romantic exponent of sex might find himself in agreement. But because the human being has this inner dimension which we call the spirit, a free and personal act necessarily involves this dimension very directly. A physical act of intimacy entails the "spiritual receiving-into-oneself" of the other person, not merely the receiving or giving of something physical or even psychological. And the acts concerned with generation are not merely pleasurable, but sacred. To the extent that they are deliberately willed, as expressions of a human love, they involve the giving and welcoming not just of the body, or a part of the body, but of the soul and the spirit.
The body is ever changing, its cells dying and being replaced. Our psychological states, thoughts, and feelings are also fluid. To locate the person, "myself", among these would be impossible, for the self is a totality that includes past, present, and future. My only access to that totality is through the spirit within, which transcends time, or at least is in contact with that which transcends time. The physical act of greatest intimacy, in which two bodies can potentially become one principle of generation and the source of a new life, should therefore be reserved for the union of one spiritual person with the other, a joining of two lives to create a new thing. Partial unions, that is unions not involving the spirit or transcending time, but involving merely the union of myself-as-I-happen-to be-now with some similar fragmentary state of the other, undermine the possibility of a lifelong commitment and communion of this kind, which the marriage vow is intended to express and make possible.
For more on sex and marriage, see HUMANUM.