As with friendships, what married couples called each other in private depended more upon the closeness of their relationship than on anything else. Mr. and Mrs. Bennet used formal titles when talking to each other because their marriage was not a close one. Emma's sister Isabella called her husband John by his first name or "my love" (and vice versa) because their marriage was a close one.
The other thing is that, particularly in books about people who are unmarried (such as Jane Austen novels), the main character is more likely than not going to hear people who are married referred to by their titles even by their spouses. To use Emma as an example, it wouldn't surprise me if Mr. and Mrs. Weston used their given names when talking to each other in private. Theirs was a love match, and they are presented throughout the book as happily married and close to each other. But Emma knows them as Mr. Weston and Mrs. Weston (nee Taylor), because she doesn't have the sort of relationship with either of them that allows for using their first names. So even if Mrs. Weston called her husband by his first name in private, she'd refer to him as Mr. Weston when talking to Emma, because that's how Emma knew him.