That's the difference of unique keys and primary keys. Primary keys will idenitfy a row within a table and they're essentially a preferred unique key. Unique keys will enforce uniqueness and a table can have as many as needed.

For me, the primary key on the marriages table should include both columns (husband & wife) or I might choose a surrogate key if the situation is correct for that. Husband and wife might be both foreign keys to a table People (and the PK for that table is a great discussion on its own).