Some of the toughest teachings of Jesus start with "you've heard it said..." The Sermon on the Mount is filled with a lot of stuff that makes us squirm -- he deconstructs societal norms, confronts the truth about ourselves, and asks us to do what is unnatural.
When Jesus spoke about divorce, swearing oaths, turning the other cheek, and loving our enemies, he wasn't just constructing a new ethic for us to follow, he was showing us the Way of Jesus. While it is true he doesn't want us disposing of our covenanted relationships, swearing on the name of God, or taking our revenge on those who hurt us, I think there's a lot of the Sermon on the Mount that tells us more about Jesus than it does about ourselves.
Because of the Law of God, he could have disposed of us. But he didn't.
He could break his promises to us since we are unfaithful, but he keeps his word.
He doesn't give us what we deserve when we offend or even strike at him; in fact, he gives us what we don't deserve -- grace.
He loves us even when we were his enemies. He loves us before we love him. He loves us even when some of us hated him.
Jesus didn't just tell us about another way of living. He showed us!
This is who Jesus is, and he wants us to follow him. That's why we fight for marriages. That's why we don't treat relationships or life as disposable. That's why we keep our word. That's why we practice mercy and live in grace towards on another. That's why we serve those who are our ideological, political, social, or economic opposites. That's why we forgive injustice done to us.
It's the Way of Jesus: another way of living.