Constraint to enforce "at least one" or "exactly one" in a database

Say we have users and each user can have multiple email addresses

CREATE TABLE emails (    user_id integer,    email_address text,    is_active boolean)

Some sample rows

user_id | email_address | is_active1       | foo@bar.com   | t1       | baz@bar.com   | f1       | bar@foo.com   | f2       | ccc@ddd.com   | t

I want to enforce a constraint that every user has exactly one active address. How can I do this in Postgres? I could do this:

CREATE UNIQUE INDEX "user_email" ON emails(user_id) WHERE is_active=true;

Which would protect against a user having more than one active address, but wouldn't, I believe, protect against all of their addresses being set to false.

If possible I'd prefer to avoid a trigger or a pl/pgsql script, as we don't currently have any of those & it would be difficult to set up. But I would appreciate knowing "the only way to do this is with a trigger or pl/pgsql", if that is the case.