Remember the Pharisees' question...divorce for "any and every reason?" (verse 3)
This was an obvious aberration of what God had in mind and Jesus points to the "beginning" and the familiar words of Genesis 2:24, repeated here, affirming the fact that "male and female" become "one flesh" and are "united" ("cleave", KJV) together, not to be ever separated. this is God's design for marriage.
The "bill of divorcement" (our divorce decree) was granted because of men's "hardness of heart"--sinfulness, unwillingness to forgive, adulteress choices, etc. It was a concession to protect the sanctity of the marriage covenant and to protect the innocent party who might otherwise be unfairly branded. The porneia concession--the "indecency" Deuteronomy 24--included more than adultery--enveloping things like incest, homosexuality, indecent exposure, etc. Pornography could be a 21st century addition to what the word semantically includes. I Corinthians 7 suggests that an unbeliever leaving a believer allows that unbeliever not to be "bound" by marriage, as well.
What I think Jesus was really doing in this passage was not so much speaking of divorce--though that is what the Pharisees wanted--but using this as a platform to affirm what marriage was to be, and what such relationships should look like within the kingdom. This, obviously puzzling the disciples now reminded of God's intended permanency of marriage, prompted them to inquire if being single wasn't a better option. Verses 10-12 provide Jesus' clear answer to that with the obvious conclusion being that marriage is our clear privilege unless calls us specifically to something different.
Which causes me to affirm today--even though I experienced the pain of divorce almost twenty years ago--that my marriage matters most. God has divinely provided me a partner for the rest of my life. He has clearly promised me He will guard this relationship as I partner with Him, and He will preserve my marriage as I submit to the principles of His Word, which guide me in how to be a godly husband.
In the sphere of all other relationships and responsibilities, I have determined that this is what matters to me the most.
So help me God!