I wonder if they'd say that less than 0.1% of users wanted to say they're married when they legally aren't married. I don't think that would fly as well as the same argument they made with names.
Somehow, I doubt it would be an issue.
I think it would be like complying with China in filtering their results where they could argue that somehow working within the system is more helpful than working outside the system. They could say that it would just give people who couldn't get legally married more incentive to become politically involved.
Google could provide a marriage API that would allow companies, like insurers, to have their system check if people are married, to determine benefits. Building law checking into their system might not target people who are attempting fraud, but removing false entries of any kind could increase the reputation of the system.