Earlier quoted context omitted.
FK consistency can't be guaranteed without ensuring the referenced rows don't disappear before the transaction has been committed. Think about it.
Yes, but it can be done without locking out reads. The only thing you need to lock against is someone changing the primary key of the referenced row or deleting the row. PostgreSQL has implemented this minimum level of necessary locking for quite many years now.
FWIW, I just tested in Postgres. Locks like I said it does:
A: create table parent(id int, value int, unique(id));
A: create table child(id int, parent_id int references parent(id));
A: insert into parent values (1, 10);
A: begin;
A: insert into child values (1, 1);
B: begin;
B: select 1 from parent where id = 1 for update;
B: (blocks)
I have specific experience of this due to use of database locks at the application level to avoid deadlocks (different lock orders) and inconsistent updates (updates based on reads across multiple tables that may have separate racing updates) by locking rows up front. For understandable schema reasons, what is logically a parent entity is the natural thing to lock, but for understandable performance reasons, FKs to the parent entity are distributed through some fairly large tables.