Many people agree: government is too big, it shouldn't be involved in private affairs, and individuals should be free to live as they wish so long as it doesn't harm others.
Many of those same people would however disagree that the government shouldnt be involved in private affairs vis a vis marriage.
I guess they think gay marriage will harm others? Id be interested to hear an argument for that position that couldn't be extended to outlaw all sorts of "amoral" behavior, thus destroying the separation of church and state and turning America into a theocracy.
And then of course many of these same advocates of private freedom of government intervention might also oppose homosexual military service, again because I somehow should believe that gay military service harms others. Even though every other western nation allows it, including Israel.
Everyone likes big government when it suits them, I guess.