What to do in this situation

Long story short…wife and I have 4 kids. Wife wants NO more. NFP not going well. She wants me to withdraw even during non ovulating times because she is scared to get pregnant. Her cycle is not very consistent making her nervous. She is contemplating birth control because I will not withdraw. If she does decide to go with birth control against my will, I am allowed to still have relations with her without it being a serious sin?

This might help:
forums.catholic.com/showthread.php?t=21761&highlight=wife+on+pill

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