Thank you for bringing up that point. I think I can clear that up. What you say is absolutely true: if God has joined the two together, then no one can separate them, that is, no one can break the bond.
That has always been the teaching of the Catholic Church, for 2,000 years, and is so today.
The difficulty comes in if God did not actually join them, because there was something fundamentally wrong which prevented it being a valid marriage from Day One.
A few "way out there" examples should make that obvious.
I know these are extreme examples, but I just wanted to make a point that not every matrimonial ceremony is valid, because sometimes there is an invalidating condition not compatible with the making of a binding sacred vow.
That is not for either of the marriage partners to determine on their own. There has to be an objective investigation of the facts, and a judgment made by a Church tribunal. The initial assumption is always that the vows are binding. That can be overturned only of there is convincing evidence that the "vows" were fraudulent or defective from the beginning, and hence not really vows at all, i.e. not binding.
We believe this is the proper interpretation of what Our Lord said about "unlawful sexual unions" (Greek: porneia)
Matthew 5:32
"But I say to you, whoever divorces his wife (unless the marriage is unlawful - porneia) causes her to commit adultery (moicheia), and whoever marries a divorced woman commits adultery(moicheia)."
Interesting! Thank you.