What's to prevent two people who've met in person from agreeing to be "married" for tax purposes? Marriage fraud is marriage fraud--an application for marriage

What's to prevent two people who've met in person from agreeing to be "married" for tax purposes? Marriage fraud is marriage fraud--an application for marriage doesn't prevent that even presenting themselves before court officers. However, there's a whole discussion on how tax benefits for married couples is in violation of the equal protection clause...