I don’t remember our priest asking “Will you actively seek to procreate in order to fulfill God’s command to “be fruitful and multiply”?” What I do remember is him asking if we will be open to children in our marriage. To me, there is a difference: If we choose to abstain during fertile times in an effort to minimize our chance at conceiving because we do not desire kids, that is not contrary to rules, laws, etc. If we conceive despite this, yet welcome and accept the child, (and raise him/her as Catholic and all the other good things people have mentioned) how have we sinned?
How many validly married Catholic couples do not intend to have children, minimze their chances at conceiving by abstaining during fertile times, yet get pregnant, then love the munchkin more than anything? It happens.
There is a distinct difference between seeking to procreate and being open to children, even though each option may be gone about in the same manner (NFP, abstaining) It seems a contradiction that NFP is acceptable in some instances, but not all.
I didn’t read into the OP’s question that she is advocating ABC or any other method to prevent pregnancy. I read it as questioning how it can be wrong to not desire to have kids, yet remaining open to it if it happens.