Brief Response:
Why has nature made man and woman so different yet so complimentary, with different anatomical structures, different physiology and psychology? Why is there a strong attraction between the sexes? Nature herself has linked sexuality with a purpose and tells us plainly that man was made male and female to bring into this world new beings of the same species. The sexual act is naturally ordained toward the begetting of species—if it were otherwise, why should there be two sexes? The sexual drive facilitates and ensures the preservation of the species. Similarly, the special pleasure attached to eating gives us an extra incentive to nourish ourselves—to satisfy the irresistible appetite and therefore fulfill the need to keep ourselves alive. To arouse and satisfy the sexual drive without regard for the natural responsibility it entails breaks the link between sexual activity and procreation and thus constitutes an abuse of human sexuality. It’s like bullimia, eating for the pleasure then throwing up to avoid the consequences. Engaging in the act and violating the act.
Why does the sexual act signify love and unite the spouses in a way that no other act does, such as a handshake? What is it that makes it not simply a physical experience but a love experience?
Is it the special pleasure attaching to it? Is the unitive meaning of the conjugal act contained in the sensation, however, intense, that it can produce? If intercourse unites two people simply because it gives special pleasure, then it would seem that one or other of the spouses could at times find a more meaningful union outside marriage than within it. It would follow too that sex without pleasure becomes meaningless, and that sex with pleasure, even homosexual sex, becomes meaningful. No, the meaning of the marriage act does not consist in pleasure. Why should the marital act be more significant than any other expression of affection between the spouses? Why should it be a more intense expression of love and union? Surely because of what happens in that marital encounter, which is not simply a touch, not a mere sensation, however intense, but a communication, an offer and acceptance, an exchange of something that uniquely represents the gift of oneself and the union of the two selves.
The greatest expression of a person’s desire to give himself is to give the seed of himself. Giving one’s seed is much more significant, and in a sense more real, than giving one’s heart. “I am yours, I give you my heart, here, take it” is mere poetry, to which no physical gesture can give full expression. But “I am yours, I give you my seed, here take it” is not poetry, it is love. It is conjugal love embodied in a unique and privileged physical action whereby intimacy is expressed—“I give you what I give no one else”—and union is achieved. Take what I have to give. This will be a new me. United to you, to what you have to give—to your seed—this will be a new you-and-me, fruit of our mutual knowledge and love.” In human terms this is the closest one can get to giving one’s self conjugally and to accepting the conjugal self-gift of another, and so achieving spousal union.
Therefore, what makes marital intercourse expression a unique relationship and union is not the sharing of a sensation but the sharing of a power: of an extraordinary life-related, creative, physical, sexual power. In a truly conjugal relationship, each spouse says to the other: “I accept you as somebody like no one else in my life. You will be unique to me and I to you. You and you alone will be my husband; you alone will be my wife. And the proof of your uniqueness to me is the fact that with you—and you alone—am I prepared to share this life-oriented power.”
In this consists the singular character of intercourse. Other physical expressions of affection do not go beyond the level of a mere gesture; they remain a symbol of the union desired. But the conjugal act is not a mere symbol. In marital intercourse, something real has been exchanged, with a full gift and acceptance of conjugal masculinity and femininity. And there remains, as witness to their conjugal relationship and the intimacy of their conjugal union, the husband’s seed in the wife’s body.
Now if one deliberately nullifies the life-orientation of the conjugal act, one destroys its essential power to signify union. Contraception in fact turns the marital act into self-deception or into a lie: “I love you so much that with you, and with you alone, I am ready to share this most unique power…” But what unique power? In contraceptive sex, no unique power is being shared, except a power to produce pleasure. But then the uniqueness of the marital act is reduced to pleasure. Its significance is gone. Contraceptive intercourse is an exercise in meaninglessness. It could perhaps be compared to going through the actions of singing without letting any sound of music pass one’s lips… Going through the motions of a love-song; but there is no song.
G.E.M. Anscombe on the natural difference between contraceptive intercourse and the use of infertile times to avoid conception: