When I look at the context of Matt 19:9, I go back to verse 7 where the Pharisees asked him about the Bill of Divorce. The Holy Spirit reminded me of Matt 5:32, where Jesus said that whoever puts away his wife, except for fornication causes her to commit adultery, and then went on to say that whoever marries her that is divorced commits adultery with her. For a while, I wondered if Jesus was not simply reiterating here, when speaking to the Pharisees, what He had said in the Sermon on the Mount, and I think the context here justifies that. Why when they asked him about divorce, does He suddenly transition to remarriage, when they hadn't even brought that up, or so it may seem. It turns out, when they asked Him about the Bill of Divorce, in fact they WERE asking about remarriage. That is what that Bill of Divorce allowed their wives to do! Even today, Orthodox Jews do not allow a woman to remarry without obtaining a "Get" from her husband. Modern Day translations were all translated by people who wore the "monogamy only" glasses, and assumed that Jesus was talking about the husband getting remarried, but the original Greek doesn't specify which spouse is getting remarried. It is implied by the context.
The beauty of looking at that verse in light of verse 7, is that the original understanding of what actually constitutes adultery, that is taking another man's wife, remains in tact. We don't have Jesus expanding that definition outside that realm, but rather, pointing out that as Rom 7:2 tells us, the wife is bound to her husband as long as he shall live, so that if while he is alive, she is married to another man, she shall be called an adulteress.