I don't think there is anything wrong in saying, "I don't want children." Personally, I think children are great and beautiful, some suffer needlessly.
I have been there. I've been the one to say, "I don't want kids" not because I'm going against any "natural order of things" in some cases, if anything, I think men do go against the natural order of things by being abusive and well I can list many things but rather not.
I have one daughter, whom I've practically raised on my own and I can say that I don't want more kids for various reasons but here are some just to name a few.
1. I think there will be a lot of suffering in the near future.
2. Financial reasons.
I'm sure there are many reasons why people don't want to have kids but not all motives are due to selfishness. Mainly, for me it would have to be that I have yet to meet a man that I am willing to carry or bring their child to life. I'm also probably not mentally emotionally and physically stable to take on another child. So simply saying, "I don't want kids" is not a sin.
Furthermore, God has the last say. The Bible does say that it is a gift to be a mother and if he thinks I can again then I will in his time.