So scrap any tax benefits of marriage. Why on Earth should the tax man care about whether a couple who live together are married or not? That's just weird and ought to be totally irrelevant to calculating how much money you should be contributing towards the running of the state — it doesn't affect how many state resources you use, nor does it affect your earning capability. If couples living together use fewer state resources (like only needing one box of recycling collecting rather than two separate ones) and it's a big enough saving that they deserve a tax cut, then they should get one regardless of whether they're married, and regardless of whether they're romantically or sexually involved with each other because none of that ought to be relevant.

Scrap any immigration laws surrounding marriage. There's not really any good reason for them to exist, and immigration authorities don't trust marriage contracts anyway. When someone wants to move their foreign spouse into a country, you often hear about them quizzing the couple on "which side of the bed does she sleep on?" and "how does he take his coffee?" (orally, of course!) so it's already the relationship between the people that matters, and not whether they have the right paperwork. So drop the pretence.

Why must anybody do the registration?