Maybe he's just saying it so she thinks it'll come eventuallyHaven’t watched this but I think he knows what to say to keep her sweet, so pulls out the “when we have kids” line to keep her on side and hanging in there! Very telling if Zoe didn’t even react, she’s probably heard it so much by now and would have reacted before, but now knows his words are empty.
or maybe he genuinely believes that she is super cool with waiting for him to be ready in maybe 10-20 years and is completely oblivious to everything outside of his own little brain