1. 48 out of our 50-and-some-odd states
prohibit gay marriage and do not recognize it. Why should the federal government recognize something that most of its states do not recognize?
2. If this question is to be on the 2020 census, petitions may start now: It takes an act of Congress to change the census, and an act of Congress usually takes all but an act of God.
This isn't a question for the Census. If gay marriage is passed in the rest of the states, it will ease into the Census questionnaire.
Is it unfair and/or inaccurate? Yes. Will it be changed by the 2010 census? No.