In a MPSIMS thread, a GQ was about to break out:
No, it’s certainly more than a tautological assertion, although cynical observers might claim that it amounts to the same thing in many places today. But in point of fact, the church does refuse annulments.
In seeking an annulment, in fact, the burden is on the putatively married person to show that there was no marriage; the validity of the marriage is to be presumed.
But there are a number of gorunds which may exist to successfully challenge that presumption.
For example, the Roman Catholic Code of Canon Law, Can. 1095, provides:
Any of these conditions might exist at the time of the marriage, but not be known to the other party – or, indeed, known to the person so afflicted by the condition. If you were unable to competently judge or evaluate the decision to marry or the ability to create a true marital relationship after the marriage, you were not capable of contracting marriage. The annulment proceeding allows the other spouse to present evidence of this lack, and, if it’s believed by the tribunal, can recieve a decree of nullity.
Can. 1101 ß2 provides that if either (or both) of the parties does not intend to honor the essential elements of marriage, or any essential property of marriage, that party contracts invalidly. So infidelity during marriage is not grounds for annulment, for exmaple, but someone who married intending never to conform to marital fidelity does not actually enter into marriage.
And so on. There are perhaps twenty different general categories of flaws that someone could point to to prove that the seemingly valid marriage they were in wasn’t valid at all. It’s not a formality to prove by any means. It requires an actual hearing, with a church-appointed canon lawyer whose job it is to argue for validity – the Defender of the Bond – and your canon lawyer, or you, presenting evidence in favor of your position.