almost there on a design

I have the following, simplified three tables:

CREATE TABLE Usrs(
usr_id serial NOT NULL PRIMARY KEY,
login varchar(32) UNIQUE,
name text NOT NULL);

CREATE TABLE EmailAddrs(
email_addr_id serial NOT NULL PRIMARY KEY,
email_addr varchar(320) NOT NULL UNIQUE);


CREATE TABLE UsrEmailAddrs(
usr_id int4 NOT NULL,
email_addr_id int4 NOT NULL,
pri_for_usr bool DEFAULT 0 NOT NULL,
FOREIGN KEY (email_addr_id) REFERENCES EmailAddrs (email_addr_id),
FOREIGN KEY (usr_id) REFERENCES Usrs (usr_id));
CREATE UNIQUE INDEX UsrEmailAddrs_No_Dupe_UsrEmailAddrs
  ON UsrEmailAddrs (usr_id,email_addr_id);

A Usr can have MANY emails in UsrEmailAddrs.

If I was only allowing ONE email per user, I'd just put the email_id in
the 'Usrs' table, and I wouldn't ask the rest of this question.

The first email addr entered must have 'UsrEmailAddrs.pri_for_usr' = TRUE
Thereafter, one and only one of the emails in 'UsrEmailAddrs' for each
usr must have have 'UsrEmailAddrs.pri_for_usr' = TRUE. I have that handled.

Because of the foreign key, no 'UsrEmailAddrs' entry can exist without
a corresponding entry in 'Usrs'. THAT's GOOD, it's handled.

Now, I want a way to prevent any entries in 'Usrs', unless there is a
corresponding entry in 'UsrEmailAddrs' with
   'UsrEmailAddrs.pri_for_usr' = TRUE. On the INSERT only, for now;
   I will figure out the DELETES and UPDATES later.

Anyone one have any ideas?