Contraception fits somewhere onto the "unchastity" spectrum of sin. I'm still figuring out exactly where. As an adult convert to CSPP, I don't understand it implicitly like I do the sins I grew up with, and it's more insidious anyway.
Pastor Petersen says, "Sexual sins are [more] destructive than other sins because they are most against what God made us to be. They are most reflective of our depravity." I've spent some time pondering why adultery carries the scandal it does, such that (for example) Dorothy Sayers writes on "The Six Other Deadly Sins" and everyone knows exactly what she's getting at. No commandment makes us sit up straighter than the 6th, even though God puts it down toward the bottom of the page. I think this is what Pastor Petersen is getting at, and here is how I understand it: adultery is the most perfect, grotesque anti-icon; the truest picture of sin. Virtue is personified in the virgin, sin is personified in the harlot. The horror of sin is that the Bridegroom finds his beloved in leather and spikes at the trashiest truck stop in town, and she laughs at his anguish. The miracle of justification is that at the marriage feast of the Lamb, the bride wears white without deceit or guile. Her mother doesn't have to comfort her by saying, "It really doesn't mean that anyway." She doesn't think to herself in shame, "I don't deserve this."
I think this is also why female infidelity causes more scandal than male infidelity (historically at least--things have evened up, and even gone too far the other direction in these utterly depraved latter days). We don't have a spiritual schema for male infidelity, because the Bridegroom is always faithful. But how well the Bride knows, and deplores on some level, her own infidelity.
Where does contraception fit into the icon, or the anti-icon? Selfishness and all that, but I still have trouble pinning it down in my mind. I wonder if this is because the Scriptural eschatological icon ends with the marriage feast, the consummation. We don't see how the story continues. All we know is that God's love is by definition incarnational: he gives us his perfect material creation, his Son in the flesh, his Son's body and blood so that we eat and drink the forgiveness of our sins. We know that our Mother, the Church (aka the Bride of Christ), bears sons and daughters of God through Holy Baptism as often as she has opportunity. These things show us that true love naturally manifests itself in real, tangible ways outside of the Lover. But somehow chastity has been reduced to sex, and huge numbers of Christians who wouldn't dream of compromising on the extramarital sex front roll their eyes or get angry when somebody suggests that babies can't be removed from chastity equation.
Help me out here. Pregnant women shouldn't pretend to think (except Reb. Mary, whom we all know to be the brains of this operation, pregnant or otherwise).