We have common law marriage here in Maryland. It does give a lot of the same rights as full marriage, but still not close to everything. And it still can not be used by same-sex couples :6 Plus I can declare my relationship to be a marriage, and have a wedding as well. Problem is that in the legal system we will be treated as strangers. Are you proposing being able to declare how you think certain parts of the law should apply to you in certain cases at specific times? (As in, yeah, "I may have been considered a witness to a crime last week, but this week I am declaring myself immune to questioning and prosecutions, and thus declare I can ignore that law")
But, valid points all. I guess we, as a people, have decided that there should be a reason for some of these permits, titles, certificates, or whatever, and now we are all dealing with our own consequences.