Paul also suggests the acceptability of mutual consent for periods of abstinence for prayer and then coming together again -- these passages don't address procreation they address sexual desire and its proper place within marriage -- the unitive function of marriage.
I think it is important to remember not to decouple the procreative from the unitive and I think it is possible to sin against one or both in how we approach conjugal relations within marriage.
It's a bit of an old forum post from another site but I think this might help answer Patrick's question as well forums.catholic.com
It basically says something similar to what Lauren brought up in that it's considered acceptable to practice a period of abstinence within marriage from what I can gather, though it doesn't seem to be encouraged to do so for long periods of time. What most of the posters on the website generally said was that the NFP method is extremely reliable when used correctly, and that people shouldn't be too concerned about the chances of conceiving a child with it's usage, as the probability is considered minimal. I can't quote numbers, but have heard great results of it from sources close to me, and it's often compared to being as effective as contraceptives.
I can't speak from experience but get the impression that when a couple gets married, and has sex using this method, they are using sex for it's intended purpose as an ultimate show of love for a spouse, and lust is something considered to be something that is involved with couples who aren't married. Is lust possible between a married couple? That's something I'm not sure of and am curious, but imagine most acts committed between a married couple would be out of love.
I think the main take away point from this is that a married couple as long as they have the intention of having some children during marriage, having sex with your spouse isn't really considered a sinful act when used in conjunction with the NFP method.