Intent has a lot to do with whether something is a sin. You BELIEVED at the time you were in a valid marriage. All marriages are believed to be valid unless proved otherwise.
It’s like if you bought a house and lived in it for 5 years and you later found out there really was no clear title. It wasn’t your house after all. Even though you went through all the steps to buy it legally and lived in it like it was your house. But the contract at its heart was invalid because somewhere along the line the house was not legally able to be sold. You didn’t know that and so are without fault.
So let’s look at an annulment. Johnny and Suzy marry. Sign the papers. They have the dress, the flowers, the church, etc. Lovely wedding on the surface. But later it comes to light that Johnny is married to Melinda in Dubuque. That completely invalidates the papers he signed with Suzy. She can file for an annulment (prior bond). Same if Johnny used to be Fr. Johnny but didn’t mention that to anyone and was still a functioning priest. Or if Suzy was Sr. Suzy but was never dispensed from her vows. Invalidates the contract.
Or if Johnny turned out to be Suzy’s half brother but dad never mentioned that he sold his sperm to make his way through college and that irresistible attraction she felt for Johnny turned out to be something else entirely.
Or if Johnny turned out to really be Jane, but never mentioned the operation while they were dating.
Doesn’t matter if they were married 10 years. If it turns out that the fundamental basis for the marriage didn’t exist, or there was fraud involved, or if Johnny married Suzy but only intended to stay till the 7-year-itch got him but promised Suzy it was forever…
Or if Johnny and Suzy marry and Suzy has baby fever for 15 years and Johnny keeps putting her off. Because he never really wanted kids to begin with.
Or say Johnny married Suzy and promised to forsake all others, but clearly intended to keep the college girlfriend on the side and a few others and Suzy found out about them about 8 years later.
Or say Johnny married Suzy because it would make him look good, but he had Bruce on the side and always loved Bruce more. But didn’t tell Suzy about that side of his life.
There are many reasons that a marriage could look valid for a while, but the inherent wrongness of it all causes it to eventually come collapsing down on everyone like a house that looked cute, but was built out of papermache or something and never should have passed inspection to begin with. It wasn’t fit for human habitation. Same with some marriages.
It works because we believe marriage creates an indelible bond between two people that can only be broken by death.
In a valid marriage that bond holds on through thick and thin till death.
In an invalid marriage, the two objects never “bonded.” Something kept the bond from taking hold. (Ever tried to use Elmers school glue on some substances and no matter how you try nothing will hold for long because of the inherent nature of the materials involved. Paper and Elmers = bond with other paper. Plastic and elmers = not so good bond with most things.)
No bond created = falls apart at the first pressure test.