First of all, the object of spacing one’s children for sufficent reason is not contrary to God’s law.
So just looking at the ‘goal’ is insufficent. one also has to look at the means.
Say for example, that one wanted to own an HD TV. There is nothing wrong with owning an HD TV.
But there IS a difference between working at honest employment to gain the money to purchase the TV, and simply stealing the money to purchase the TV.
In each case, the end result, the owning of an HD TV is accomplished, correct?
The MEANS are different.
So we know must look at the means of the use of Artificial Birth Control (ABC) of any type, and NFP (we will leave aside the fact that NFP can be used to CONCIEVE at child for now)
Using ABC involves an element of rejection of the other person, It is a rejection of their natural (God given) fertility. It is a statement that you love the person, just not ALL of the person.
What do you think a spouse would say of you said to them, “I Love you, but I reject your face, please wear this brown bag over your head before I will make love to you”. Does anyone really think that such a statement is a loving, caring thing to say. Does it show love for one’s spouse.
But ABC is making the same statement in regards to the spouses fertility. That you do NOT love all of them, that you insist that they cover up their fertility before you will make love to them.
Rejection should not be a requirement for the marital act. It is contrary to the very nature of the marital act. It is a rejection of the fertility that God gave them.
NFP, on the other hand, gives full acceptance to the fertility that God gave each person. There is no rejection in the marital act, only full acceptance of the entirety of the spouse.