A simple reading of the Gospel might suffice:
Matthew 19: 3-12
And there came to him the Pharisees tempting him, and saying: Is it lawful for a man to put away his wife for every (RSV and many others: 'any') cause? Who answering, said to them: Have ye not read, that he who made man from the beginning, Made them male and female? And he said: For this cause shall a man leave father and mother, and shall cleave to his wife, and they two shall be in one flesh. Therefore now they are not two, but one flesh. What therefore God hath joined together, let no man put asunder. They say to him: Why then did Moses command to give a bill of divorce, and to put away? He saith to them: Because Moses by reason of the hardness of your heart permitted you to put away your wives: but from the beginning it was not so. And I say to you, that whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery: and he that shall marry her that is put away, committeth adultery. His disciples say unto him: If the case of a man with his wife be so, it is not expedient to marry. Who said to them: All men take not this word, but they to whom it is given. For there are eunuchs, who were born so from their mother's womb: and there are eunuchs, who were made so by men: and there are eunuchs, who have made themselves eunuchs for the kingdom of heaven. He that can take, let him take it.
They ask Him if divorce is lawful, He contradicts them immediately and explains much, and in large, His answer is NO.
They then ask why did Moses allow it? They ask this because Christ said NO.
"From the beginning it was not so". Since this is the Perfect Covenant He is restoring true order to marriage.
The disciples protest as well at His hard teaching, like many Catholics and Eastern "Orthodox" do. And Christ clearly says celibacy is better.
This is also a good time to point out what the Magisterium is and why it is useful.