My H and I are laid back too and would be ok being called by first name. I just had this exact conversation with a group of friends and my friend's husband wants to be called [prefix] first name...so either Mr First name, Coach first name (he coaches a sport team that many of our boys are on). My other friend wants to be Ms. first name.
When I was growing up, I called my friends' parents Mr/Mrs. Last name and my parents' friends by their first names.