I don't get it, why is he offended? She's just stating facts, he decided to not have his own kids, it's not some huge deal like saying to a cripple "you'll never walk again", and he was on the offence first by saying that she's insanely overreacting, prompting her to defend herself.
Like, you picked to take care of another man's child, naturally it won't be the same as having your own and raising a kid from the moment they were born. Why does he act like she gravely insulted him?