The biggest case of discrimination regarding marriage is discrimination against the unmarried. I could go for getting the gummint out of the process if they and everyone else would/were required to get completely out of the process - no more tax breaks for marrieds, no health insurance discounts for "and family", no reduced auto insurance rates for married people, etc. Make a person a person with each one having exactly the same liabilities and privileges - no special treatment for anyone, period - and be done with it.
I would agree with all except auto insurance rates. Rates are supposed to be based on probability of risk - married = less risk = less cost.
I also advocate airlines charging passengers by the pound, as opposed to per person.