Hard teaching for the ears of the 21st century in today's Gospel reading!
But it was hard teaching for the ears of the Pharisees as well in the 1st century.
Our human nature is wounded by sin and so we can find ourselves not receptive to God's reasoning and plans. There can be a response to question it and even to ignore it, and some may reject it.
Thankfully, we also have the effects of grace which turns our minds, and also our hearts, towards what God speaks to us, because we sense in it not only what is true, but also what truly satisfies the human heart.
Marriage, sexuality and human relationships lived according to God's plan will only bring peace and human flourishing.