Castle exhaled slowly before answering. "If I'm being honest, Kate, if we never had children... I would be alright with that. I did get my chance to be a dad with Alexis and she was a wonderful kid - an experience that probably won't be repeated because even I'm not that lucky. Do I want to have kids with you? Yes, if that's what you want. Am I going to be devastated if we don't?" Here he shrugged. "Honestly, no. I love you and I'll be happy for our time together."
He did agree that she would have to make her decision soon. He'd rather not be 80 and still chasing a teenager around. Realistically, Castle knew that the burden of a child would fall more upon Kate as the mother. She would have to carry the baby for nine months, breastfeed, etc. It's not that he wouldn't help her in every possible way, but a mother was always so much larger than life to a child as compared to the father. Castle wanted her to be comfortable with and ready for that.