I don't think you are selfish. I think you have put more thought into this than many parents do into actually having children. It's selfish to have children for your own gain. If you ask anyone who has children why they have children, there answer often starts with "I wanted ..."
I have written a few pieces here on medium about being childfree.
The Pressure to Have a Baby Almost Broke Me
The Hypocrisy of a Childfree Dog Parent
I Miss You