and then go speak to a lawyer about the validity of a marriage contract that is not state approved (not on their special paper)
We're not interested in the state's interpretation of the validity of our contract, but I'll be happy to take a look at lifeonbitcoin.com.
Seriously.... you should at least see to it that your marriage is officially recognized by at least one jurisdiction that your government and locals will see as somehow valid and official. Or see a lawyer about other options.
The concern is about various situations where not having an officially recognized marriage can be extremely inconvenient or troublesome.
If you can prove marriage; you have additional legal rights in some situations, such as with kids -- guaranteed right of both parents to act as guardians, pick up kids from school, shared property rights, etc, or if spouse is temporarily incapacitated or in hospital: without proof of marriage/family relationship, or legal documents, there is no authority to direct their care.
The blockchain marriage is a cool idea and all, but I would advise you to not take unnecessary risks in giving up legal privileges couples with the legally recognized state documentation ordinarily have.
Another issue is that if your marriage is not recognized then you will not be able to reap the tax benefits of marriage