do foreign key checks lock parent table ?

If you try the example I have below - the second transaction will block
waiting for the first one to complete.  This doesn't seem correct to me.
Is this a known problem / feature?  If I create the table w/ a deferrable
intially deferred foreign key - I don't get the problem.  But this is a
very unexpected default behavior ( At least to me :) ).

create table parent
     (key         integer primary key,
      description varchar(50) not null);
create table child
     (key       integer primary key,
      child_key integer not null references parent(key),
      description varchar(50) not null);

insert into parent values (1,'Parent Record #1');
insert into child values (101,1,'Child Record #1');
insert into child values (102,1,'Child Record #2');


// From one psql
begin;
update child set child_key = 1 where key = 101;


//second session
begin;
update child set child_key = 1 where key = 102;