My understanding is this…
There are two problems with contraception and artificial birth control (probably more, but two that I can think of right now). The first deals with the full gift of self and the second deals with openness to God’s will.
First… full gift of self. Let’s assume we agree that sex is reserved for marriage and is a complete gift of oneself, body and soul, to another (in holy matrimony). Sexual intercourse is the highest expression of love between two people and requires the marriage commitment. It is a visible manifestation of God’s love and it is life giving.
In order for sexual intercouse to be meaningful, that is, in order for it to express complete sacrifical love for another, it needs to be a complete gift of oneself and open to life. The key word is complete. It’s not enough to say, “I give some of myself to you.” When you engage in intercourse using contraception, that is precisely what you are saying. You are literally blocking part of the physical gift from entering the other person. When you use chemical birth control, you are killing that gift, denying the possibility of life. The man’s gift to the woman is killed and essentially worthless, since it is incapable of creating life.
With NFP, however, you are giving that gift, in it’s entirety, to the other person. There is nothing in the way of the gift, it is complete.
The second is tied to the first. It is very much correct to say that the intention has much moral bearing on an act. The intention in question here is the openess to life.
During an act of intercourse, God is working and may be willing that life come from that act. When a couple uses contraception and chemical birth control they deny the possibility of life. Effectively, the couple is stating, “not only do we not give all of ourselves to this other person, but we are also not open to the possibility of life, to God’s will.” Now if God wants a baby to be created, well, then perhaps he will cause the condom to break or some sperm to reach the egg without dying anyway. But that’s not the point, the point is that during the intercourse the couple is saying, to the possibility of life and therefore “no” to God’s will. The intention can’t be openess to life with these two options.
With NFP, however, it is intrinsically open to life. The intention in this case is, “Lord, it is not convenient right now for us to have children, but we’re still open to the possibility. We’re going to give to each other fully and if it is your will that children come about, so be it.” With NFP, the couple simply does not have sex during a certain time. Just because they have sex in the infertile period doesn’t mean it isn’t open to the possibility of life. The act is intrinsically opening the door to God. There is nothing that says they have to have sex during the fertile time. God does, however, decree that sex must always be a complete gift of self, or it is a selfish misuse of the gift of sexuality. Contraception and birth control do not allow this comlete gift and so must be wrong.
This is my understanding and the distinctions can sometimes be subtle. If this is in any way unclear, let me know and perhaps we can puzzle it out.