Castiel isn't certain he likes what she's saying. He cannot explain why, for the idea of a wife and children is not an unpleasant one. In fact, it is expected of humans once they reach a certain age, but he doubts very much that it is even possible. For one, he is still an angel inhabiting an empty vessel and he knows that he can tell no human woman this without frightening her or causing her to believe he's insane. Yet he does not say this since the mood is meant to be light.
Instead, he watches her as she simply enjoys the night air. It is beautiful it is simplicity and he is aware that though his sense of aesthetics may not be fully developed, he knows that Anna - her smile and exuberance - is breathtaking to behold. It leaves him with an undeniably pleasant feeling and her confession is a sign of trust. That he's earned it back, despite his misguided past actions against her. It is a comfort.
He understands her dilemma. When they were a part of the Host, they were accepted. Cared for by their siblings. Now, they were not only public enemies to be destroyed on site, but they are completely cut off from that security. "I...believe it was our Father who brought me back," he stated slowly. "I also have every reason to believe He was responsible for my awakening the day Michael nearly killed you. He may not want our contact, but He is there...assisting in his own way."
Castiel did not expect her to agree with him. Considering Anna's past feelings about God, it was likely she would vehemently disagree.