You go off the rails in your first sentence - "age-old Catholic doctrine"    Guess what - the letter is based upon a recent interpretation (over last 50 years) and actually refutes the moral theology manuals and structures that had been in place since Trent.  What you have with these signers is a very thin veneer of historical moral/sacramental understanding.  Rather, they pick up on the anti-modernism kick at the turn of the 19th century and enshrine this as perpetual catholic moral tradition - SAD.