Or, more likely, Americans will grumble a bit but do nothing. And the legal definition of marriage will change for all time.
This only would open a box to more social ills that would result in the end of this society
I'm not sure I see how. Allowing adults to enter into contracts in no way requires anyone else to do anything or to accept any type of behavior, since the contract is only binding upon the parties thereto. On the other hand, if the legal definition of marriage is changed by the judiciary to include gay marriages, the rest of us will be required to also recognize it: companies will be forced to extend the same benefits to all married couples, anti-discrimination laws that outlaw discrimination against married people will apply to gay marriages etc.