Yeah if you're against it, then it should be an easy decision.
Don't place anyone in a situation that they aren't sure of. Especialy children.
I am really one who advocates for a healthy home for children. If anyone has issues with this, it will not be healthy for the children.
Smooth things over with the couple as simple as "we aren't ready and when we are, we'd like our first child (or all depending) to be ours. We are happy with this relationship, but this is a step we aren't ready for"
How would you handle it if you weren't married and the one your were dating for a year said, I want you to parent my child? I'd personaly run, but that's me.
I question why they would even bring this up.