I think the difference is that when someone uses natural family planning to AVOID having a baby (choosing the timing), they MUST be open to at least the possibility of still having a child anyways -- after all, timing isn't everything. But more to the point, a lot of people seem to use NFP to ENCOURAGE pregnancy -- by trying for a baby during the MOST fertile times of a month. The idea is to time your cycle so that you stand the best possible chance of getting pregnant when, ideally, you want to get pregnant..... when it comes down to it, as a "method", NFP seems to be more open to new life happening.
Condoms, when used as contraception, do NOT have a 100% success rate either (neither do birth control pills, or anything else combined) -- trust me, I know.... and yet the use of condoms as a "method" is not open to new life happening. People who use condoms to prevent pregnancy do NOT want to get pregnant at that present time, and are deliberately taking steps to prevent it.
And honestly, in our culture, condoms are seen as some odd sort of guarantee: as in, "we're being safe, we're being responsible, we don't want kids right now so we're using condoms -- and/or foam -- and/or pills -- and/or insert anything else here." The implication is that BECAUSE people are using these things, they think they can have sex without having to be responsible for a baby.
In the last few weeks, I have PERSONALLY heard the following argument: "why SHOULDN'T you get an abortion? We were careful, it's not like we weren't using protection. I'm not ready for kids. We shouldn't have to DEAL with that." -- as though because we were "good" and used condoms and other contraception, our "reward" should be not having to raise a baby.
And, quite frankly, I found the argument to ring quite hollowly. I honestly wish to God, every day, that I saw the fault in this empty logic a long, long time before I finally did. I can only thank God that He opened my heart to see the truth, and to see that the ideas our culture puts out there can be dangerous.
Edited to add: don't get me wrong on one thing -- I LOVE the little life growing inside me. I love babies. I always have. But I used to mistakenly believe that artificial contraception would actually work effectively, and that it was right or okay to use, in order to justify having a sexual relationship outside of marriage. I can't speak to the notion of using it IN a marriage, as I haven't ever had that experience. But I hold the view and the belief, now, that it's better to be open to life happening if you're going to choose to have sex. Constant fear of pregnancy isn't healthy for anyone.