Some Catholic thinking about sexuality goes something like this: There is a sacramentality to the marital act. The giving of self to another, and the seeking of the other's receiving and reciprocation, is a symbol of the union between God and his people, and maybe even has a Trinitarian significance of imaging the self-giving and generative nature of God. This, in turn, suggests that mixes up holding-back with self-giving and expresses love in a context in which one is fighting against one's generative striving, producing a parody rather than image of Trinitarian love.
I think it is worth thinking about the sacramentality in assertion as well. In assertion, one reaches out to offer one's testimony, inviting the other to receive one's testimony in trust. This is some sort of an image of the Father's sending his Word to us in time, to offer his truth to sinners, some of whom will accept him with trust and some of whom will reject him. It is even an image of the Father's eternal generative speaking of his Word. There is a kind of sacrilege, then, when one knowingly asserts falsely. One is parodying the life of the Trinity rather than imaging it. One is being faithless to those whose faith one is inviting by one's testimony.
It does not matter that the end for which one contracepts or lies is good. To do something that parodies the life of the Trinity is wrong regardless of what further end it subserves.
Of course, in both the case of marriage and speech prudence is called for. One can legitimately refrain from marital union or from assertion when it would be imprudent to generate children or communicate truth. To do that does not image, in the relevant respect, the life of the Trinity, but also does not parody it. We cannot image the life of the Trinity in all its respects anyway--we are but finite. And when refraining for the sake of a virtue V, say, the virtue of prudence, from imaging the life of the Trinity in one respect, we are thereby imaging the life of the Trinity in a respect shown by V. And even when cannot image the life of the Trinity in some respect, we can and must refrain from parodying it.
There is a cost to doing the right thing here, and the tragic cases are where the cost will be borne by others. But that is how it is in our world. The cost of Christ's birth was borne by many a newborn and his family. Yet we honor them as the Holy Innocents, for they have received the salvation that came through Christ's incarnation. And likewise the benefit of the Christian's imaging in truthfulness and sexual integrity the life of the Trinity extends mysteriously to all by the communion of the saints.
This is some sort of a reply to Janet Smith's article on lying in First Things, which saddened me much as I greatly respect her work in sexual ethics.