In speaking against divorce, Jesus said that by marriage the man and woman become one body; and so, we should no more separate a man and a wife than we would think of separating one side of a human body from its other side.
However, Jesus did recognize the fact that God let Moses grant divorce in the case of people with hard hearts. So, under some conditions God seems to allow divorces.
That brings up the matter of Catholic annulments. A group of scholars approached Pope Paul VI in 1967, asking him to see that there were cases in which annulments should be granted. They began by asking the Holy Father to consider the words used by the priest at the beginning of the marriage ceremony.
The priest asks, “Have you come here freely, and without reservations, to give yourselves to each other in marriage.”
From that we see four things needed for a marriage: 1. They had to be free. 2. There could be no reservations, 3. They had to give themselves, 4. They had to be committing themselves to a true marriage, not to a lesser arrangement.
Those experts convinced Pope Paul that he should allow for annulments to be issued if one or more of those requirements had not been met. The Pope allowed every diocese to open a court for considering whether or not an annulment could be declared on that basis.