Think of it this way.
The problem domain exists forever. Your class definitions will reflect the eternal features of the domain.
The relational database is today's preferred persistence mechanism. At some point, we'll move past this to something "newer", "better", "different". The database design is merely one implementation; it reflects a solution architecture more than the problem domain.
Consequently, it's domain first. Classes reflect the problem domain and the universal truths. Relational database and ORM come second and third. Finally, fill in other stuff around the model.
