Doesn't this set NFP to the same level as contraception?
To the question "Whether it is a mortal sin for a man to have knowledge of his wife, with the intention not of a marriage good* but merely of pleasure?," St. Thomas Aquinas answered:
…if pleasure be sought in such a way as to exclude the honesty of marriage, so that, to wit, it is not as a wife but as a woman that a man treats his wife, and that he is ready to use her in the same way if she were not his wife, it is a mortal sin; wherefore such a man is said to be too ardent a lover of his wife, because his ardor carries him away from the goods of marriage. If, however, he seek pleasure within the bounds of marriage, so that it would not be sought in another than his wife, it is a venial sin.
*The marriage goods are children, fidelity, and sacrament.
Does the child always need to be expected?
Yes, it is unjust for the child who could be conceived if one was not prepared to support him.
This is also the reason acts of adultery or fornication are sinful; it's not because the sexual act is bad (on the contrary, it's good and can even be virtuous/meritorious), but because it is unjust for the child who could be conceived. St. Thomas Aquinas, the greatest doctor and theologian of the Church, says this in Summa Theologica II-II q. 154 a. 2 ad 4:
Simple fornication is contrary to the love of our neighbor, because it is opposed to the good of the child to be born, as we have shown, since it is an act of generation accomplished in a manner disadvantageous to the future child.
Also, a child can be conceived even while contracepting; no contraception method is perfect at preventing conception.
I know there is nothing wrong to have an unexpected baby or to postpone having a baby.
It is "disadvantageous to the future child" either way, even the latter; postponing means you are going to be older parents, and that can be as "disadvantageous to the future child" as not being prepared to raise an unexpected baby can be.
Don't the couples' always trying to avoid pregnancy changing the meaning of NFP use? Didn't it become to be 'open to new life but always try to prevent it'?
Yes, recourse to infertile times as a means of lessening the chance of conception must only be used for grave reasons. For example, if the husband demands payment of the marriage debt and the wife has a disease that would make being pregnant life-threatening or the husband threatens to commit adultery.