It's really not that difficult.
And the law is not as clear-cut on those issues as you might think. For example, I am the sole surviving member of my mother's immediate family, but I still had to fill out boilerplate for power of attorney. And, because she didn't check the "Medical POA" box, I still cannot make medical decisions for her. Many people die without wills and its up to the state court to decide if any surviving relatives get anything. The concept of "community property" owing to wedlock varies widely by state. But, my company has a boilerplate for all that stuff (POA, will, living will, etc.) that takes about 10 minutes to fill out. Plus, the document's a heck of a lot easier to change than one's marital status.
All in all, I think probably the best solution is to just abandon any state interest in marriage and leave it to rabbis, priests, reverends, preachers, the FSM, etc.