I mean, if you don't want to drive anymore, do you have to go to court to "nullify your drivers license"? Of course not.
But you do have to take legal action to get out of a legal contract - like a marriage.
I completely support removing the government from the process and keeping marriage a purely social and private institution. Absolutely, 100% agree. However, currently, it is a legal contract that discriminates based on gender. And as long as it is a legal contract, then it cannot be allowed to discriminate based on gender.