Here's a juicy sample of Msgr. Burke's writing:
Why is the act of intercourse called the conjugal act? Why is it regarded as the most distinctive expression of marital love and self-giving? Why is this act - which is but a passing and fleeting thing - particularly regarded as an act of union? After all, people in love express their love and desire to be united in many ways: sending letters, exchanging looks or presents, holding hands... What makes the sexual act unique? Why does this act unite the spouses in a way that no other act does? What is it that makes it not just a physical experience but a love experience?
Is it the special pleasure attaching to it? Is the unitive meaning of the conjugal act contained just 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 conjugal act may or may not be accompanied by pleasure; but the meaning of the act does not consist in its pleasure. The pleasure provided by marital intercourse may be intense, but it is transient. The significance of marital intercourse is also intense, and it is not transient; it lasts.
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 just 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 two selves.
Here, of course, it should not be forgotten that while two persons in love want to give themselves to one another, to be united to one another, this desire of theirs remains humanly speaking on a purely volitional level. They can bind themselves to one another, but they cannot actually give themselves. 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 particular is much more real, than giving one's heart. "I am yours, I give you my heart; here, take it", remains mere poetry, to which no physical gesture can give true body. 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" - 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 come 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 express 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 true 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 with you alone - am I prepared to share this God-given life-oriented power".
In this consists the singular quality 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 true 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.