Duke had mentioned once that he was used to partners who didn't want to use "official" words to describe their relationship. He wasn't letting himself think about how in the past, that was usually because they were ashamed of dating him.

(He was pretty sure Lucifer was at least a little ashamed of what Duke was proposing here, yeah.)

"Well, you know, he's used to much longer time spans than us mere mortals. It takes him awhile to adjust."