"I'd be honored to help Naegi-sama with something like that!" Nagito chirps; he did help a little with Makoto's family, though Nagito still doesn't know why Makoto forgot that in the first place. "I suppose it couldn't hurt to offer my assistance if he'd like it. I'm not sure how reliable my memory is, but I can always write down whatever he wants me to keep track of for him!"

Nagito is ignoring the marriage joke for his own health. He doesn't think he can safely deal with that without imagining it, and if he imagines it he might die. Again.