There are a few different ways of understanding how artificial means of contracepting is in opposition to God’s plan.
1. The Bible presents children as a gift & blessing from God:
(Psalm 139:13-16; Genesis 28:3; Genesis 4:1; Proverbs 17:6; Jeremiah 1:5; Galatians 1:15)
This does not mean that because a Married coupe is unable to have children that you are therefore unworthy of God’s blessings. God may bless you in other ways, as long as you are faithful to Him.
2. Scripture talks about being open to life and of a responsibility for us to cooperate with God’s plan:
(Genesis 1:28; Genesis 38:8-30; Leviticus 18:22; Deuteronomy 25:11-12; Hosea 9:11; Malachi 2:15; Matthew 19:6; Romans 1:26-28)
3. Other verses to consider when talking about the Marrital act:
(1 Corinthians 7:5; Ephesians 5:25)
There are many debates & misconceptions (no pun intended) about NFP, however I will leave that up to you to research the various types of natural family planning and the varying arguments for and against the use of NFP from a Catholic perspective.