(TWO)

Your first para is exactly correct. In one of the two texts (sorry no bible handy), the Greek is now being understood by scholars as Jesus being asked to take sides in a then current Jewish dispute about divorce. 

The phrase "for any cause" references one rabbinical side with a new fandangled theory--a man can divorce his wife for "any cause," no matter how flimsy by giving her a "git" (certificate of divorce). The other side held the traditional view in the law of moses (Leviticus somewhere) that both men and women can divorce their spouse for adultery. 

Jesus adopted the traditional view, so according to this new reading, Jesus did not assert our current magisterial position, but allowed divorce for adultery. Again, I don't have a bible handy, but I believe this interpretation reconciles this passage with the other passage about divorce in which Jesus specifically mentions adultery.