Ok friends, help me understand!
1) The
goal of NFP is to
avoid conception through a knowledge of biology.
2) The purposeful avoidance of conception is called contraception.
So how is NFP ok?

Would the Fathers have ok'd NFP? No, actually such methods (in admittedly archaic forms) were among the first methods to be talked about in a negative light (ie. not allowed). Pro-creation was the key to sex (not necessarily marriage) for the Fathers. Curbing lust was another thing that some Fathers were concerned with, but it was always with pro-creation in mind. The Fathers simply didn't allow actions that would fulfill sexual desire while leaving no room (or trying to avoid) procreation. Augustine admitted that he didn't know anyone who thought about pro-creation every time they had sex, but that was the
goal set by the Fathers, nonetheless (sort of like other nearly unattainable goals we must work at, such as "be ye perfect")
Therefore, oral and anal sex, masturbation, etc. were not permitted. Pro-creation wasn't, of course, the thing that had to be kept in mind for infertile couples, and infertile couples (or elderly couples in general) did not
have to cease from have sex (though they were counselled to do so, and some Fathers thought it was shameful for those beyond the child-bearing years to have sex, but these were usually the Fathers who considered procreation the only
proper use of sex).
But back to NFP, what the Fathers were against was the intentional prevention of conception. Fasting from sex during times appointed by the Church (e.g., Lent) was not an
intentional avoidance by the couple, so it was ok. Skipping having sex during the times of the month that the woman was deemed to be able to conceive, on the other hand, wasn't allowed. (more quotes will follow, once I get everything organized).
The Fathers did not draw such sharp distinctions between "natural" and "mechanical," between "passive" and "active". Perhaps such a distinction needs to be made today; if so, I'd love to hear it. NFP seems like the best of the worst, so to speak; the least of the evils, if contraception has to be used (and when I say that, I am not just following Augustine

).
As to what acts are allowed and what aren't, I think it depends on whom you ask. Some penitential literature gets quite explicit about what is and isn't aloud (check out
this page to get an idea of what I mean), while other early Church texts are pretty vague. Essentially, most Fathers went along a basic line that anything that avoided or would prevent conception would not be allowed. A possible second reason for sex for some fathers was the curbing of lust, but in their mind, doing sexual things that didn't allow for conception was not curbing lust, but was in fact giving into and indulging lustful wants. They did not see oral sex and masturbation as a "release" or "relaxing" action, or one that could be used in the bedroom.
I've read Fr. John's paper (he was kind enough to send it to me a few months ago), and I very much appreciate his effort. He told me that he had gotten a mixed reaction, but that Fr. Thomas Hopko had told him that everyone he knew who had deeply investigated the issue came to the same conclusions (more or less) as Fr. John. On the other hand, Fr. John also allows for NFP, if I remember correctly, one reason that I'll ask him to critique what I write when I write it

(I know my effort will be insignificant, but he has links to things like Indiania List posts on his site, so I know he's willing to consider/read things from even peons like me

)
Anyway, I know my stance on this will be accepted by... oh, about... .01% of the Church, lol. This is how I see the Fathers as teaching, though, and I see nothing that would make things different today (unlike other areas of praxis, where our context has necessitated changes)
Justin