

Basically, cognitive dissonance. They simply refuse to believe the truth, and make up their own fantasies in which everything is backwards from reality. It makes them feel like they have some power over their life, and/or that they’re special.
99% of the time people like this are just deeply freaked out at the idea of a cold uncaring universe with nobody in control.
The “death do us part” thing is a tradition, but marriage is a legal status. Not everyone is going to follow that tradition, and surely you wouldn’t argue this ought to bar them from the legal status