1. There IS already such a "contractual registered agreement"; it's been around longer than christianity, and its name has always been "marriage". Look up the English concept "common-law marriage", for instance, and reflect a bit about how old it and the institution it describes might be, compared to christianity among the speakers of English; and then on why it was coined in the first place. (Hint: To describe people who felt they "were married" just like their forebears had always been, without these new-fangled ceremonies the new faith was trying to claim were mandatory, in order to usurp the regulation of the domestic life of the populace as its sole prerogative.)

2. Yup, exactly. And if they can, then so can (other) religious nutcases, too.