Far be it from me to be critical BUT:

Isn't your child number 1 here? It's highly fashionable not to get married these days - blowed if I know why, especially with a child.

And you're already posing the question "If we were to split..." and "Assuming we stay together..."

Stop thinking about your wallet. Start thinking about your child - this is just the beginning you'll be paying for the next 21 year (and doubtless beyond). But surely you knew that...