The Magisterium has 3 elements: dogma (like the Trinity and Resurrection), natural law (which is up for revision by advances in evidence and reason - the best argument wins or it is not natural law) and practice. Marriage is in both natural law and practice and it can and has changed. Indeed, according to some on this page, this teaching was based on inside baseball discussions between to rabbis. Leaving the latter out of the real discussion leads to a false result. Too bad this was not brought up at the Synod.