I'd like to see this data (including the exact wording of the question). I don't buy it for a minute.
BTW, for what it's worth, I am of the opinion that the government should have nothing to do with marriage except allowing individuals to designate heirs, 'next of kin' status, etc., to whomever they want -- most of which can probably be done right now via wills, contracts, powers of attorney, and so on.
A very good observation, questions on polls can be twisted easily to alter the results, and often are! You can find that here, on the Gallup website:
Most Americans Approve of Interracial Marriages
The question asked was: "Do you approve or disapprove of marriage between blacks and whites?"
According to their data, 1991 was the first year more respondents answered "approve" than answered "disapprove".