But in those days, adultery was punishable by stoning, so what would they have understood his words to mean?
The woman who was caught in adultery, they didn't say to her husband, "well we have proof of her unfaithfulness, we will allow your divorce"; they dragged her out to stone her to death. If any other man had accused his wife of adultery and had proof, she would have suffered the same fate.
Until the angel appeared to Joseph, he was going to divorce Mary because he did not want her to be disgraced when it was known that she was pregnant, Matthew 1:19.
Because they understood that adultery was punishable by death, I am certain they would have understood Jesus' words to mean that divorce is not good at all. And he said this, because they commented that Moses had allowed men to write their wives a note of divorce - treating it casually; ending a marriage, and dismissing their wives, by leaving a note on the table. It should not be like this, Jesus said - God made men and women in the beginning, put them together and intended for them to be one flesh.
He is telling the men to treat marriage seriously and not as if their wives are property to be discarded when they wish.
Maybe.
What if a couple divorce, live apart and neither remarry though?
The thing is that the Bible doesn't address some of the scenarios we can have today, so what are we to do, or say, in those situations?
What of violence within a marriage?
What if one half of the couple starts using drugs or alcohol - and gets them into debt to pay for their habit?
What if one, or both, people have made vows before God but don't actually believe in him or mean them?
What if one turns to another faith and rejects God?
What if a man walks out on his wife for his mistress, and 5 years later wants a divorce, which (in the UK at least) he could do without her consent? Does the woman have to remain single, never look for, or enter into, a loving relationship again because of something that was not her doing or her will?
Do we tell people that God expects them to endure violence, rape, bad treatment, broken vows, debt etc forever, because they made a mistake and married someone who was, or became, the wrong person? How does that speak of a God who IS love? How does that encourage people to want to know, and worship, him?
We can't say what the Bible teaches in these scenarios, because it doesn't.