Gene writes:
"Umm... NFP is stupid. Hate to break it to you, but it is a form of birth control that Catholics should be against. Why? Because it undermines Catholicism's own understanding of why birth control is bad.

My understanding is that birth control is bad because it is the purposeful evasion of a natural gift of God (ie: ability to beget and have children).

But, NFP is the same thing! If a husband and wife are having sex, but telling eachother, "well, we can't do it on the 10th, because the missuz is ovulating that day," they are purposely avoiding that natural gift of God, ie: children. Your only objection to the other methods seems to be not that the attitude that it breeds in the couple (ie: desire not to have kids) but rather that they are more effective than NFP!

More evidence for why Catholics' support for NFP is morally corrupt:

Here is how a dialogue usually goes between a hardcore Catholic trad and someone who really really likes sex at all times:

Sex-Maniac: "Well, Catholicism is stupid because I can't use a condom."
Trad: "Well, but there's NFP! And it's really effective too! You know, there's all these modern methods now of telling when you're more likely to have kids and when not!"

If you are touting NFP as a good enough substitute for condoms with that argument, you are essentially promoting NFP as a birth control measure.

The defense of NFP is often based on "being open to having children." But, that's just false. Clearly if you were, you'd just be having sex when nature calls and not when the woman is not ovulating. And if you are still open to having children with NFP, then you are also with using condoms. Why? Condoms are only 95% effective when used properly. And if the person using them promises to oneself/God/etc not to get an abortion if the condom fails, then then, that's still being open to having children. Sure, NFP is the most ineffective birth control method out there (though it is still pretty good... nearly on par with condoms, from what I understand). But still. There's only a quantitative not qualitative difference between them and condoms."
Finally Gene adds - "in the beginning of the previous comment, by birth control, I meant contraceptive."

Gene - You're absolutely right that NFP can be used with a "contraceptive mentality," and if it is used in that way it's worthless. Some Catholics are aware of that, but given the the lousy state of understanding in this area it's not surprising that it's still around. I think what Catholics who make the "but there's NFP" argument are trying to get at is the idea that one's choices are not between contraception use and "all babies, all the time" if one wants to have any sort of sexual life in marriage. Because contraception is the dominating answer, and few people have any sense of how their bodies work anymore, it's easy to believe (as I used to) that if you weren't using contraception you could get pregnant basically anytime. That makes people think that the Church is being irresponsible and failing to take into account the many legitimate reasons a couple would need to postpone children.
In regard to the "contraceptive mentality" issue, however, NFP is better than contraception because it makes it more difficult to conduct your sex life on autopilot. If you're using contraception, you fall into a habit of it and don't often reflect on whether you are still disinterested in raising a family. Because NFP requires a bit more thought and involvement, it's harder to ignore the fact that you are always making a decision for or against children at this time.
To your second point on openness to having children, what the NFP process is getting at here is the difference between frustrating the fertility of the woman when she is ovulating vs taking advantage of the fact that she is not fertile. It's less an inquiry into the mindset of the couple but a question of whether they are working against existing fertility or not. I'm on my way to work now and don't have time to explain this point better - maybe someone can help out in the comments box. But your questions are good ones, absolutely, because NFP can be abused like anything else. For example, financial stewardship - we should all save enough money to take care of our families, etc. But someone could use that principle to justify a lack of charity towards others and never helping anyone else ever, which would be wrong although they were appealing to a legitimate principle.
Also I enjoyed your dichotomy between Catholics and sex maniacs. :)