If you don’t care if your marriage is legal, then don’t comply with the law, it is that simple.
This would merely mean that there is no such thing as marriage, which is silly, all societies have to deal with marriage law, and always have.
Marriage is an implied contract. And with implied contract there has to be a set of rules to adjudicate the inevitable differences that come up between two people.
Like, who is responsible for the care and support of the children who re the result of this union? If one or both parents abandon the duties, at what point does the greater community step in?
All these disputes, and other matters like inheritance, or property ownership, or even separation of parties with irreconcilable differences that may very well lead to mayhem or murder, are very much in the wider interests of the community. At one time, these disputes were settled within the confines of the religious authority figures, but with the deep schisms that have appeared between factions of the religious authority, this was no longer a reliable agency to settle differences. Religions sometimes are not very good at making judgments on a wide range of human mischief and malfeasance.