This statistic (47% think male + male = marriage) only illustrates the state of public education today. The schools desperately need to restore "marriage = man + woman --God---> baby = posterity" into the biology, history, English, social science, civics, and various other curricula. High-school graduates should understand the definitions of simple words like "marriage," the basics of mammal reproduction, and the existential role of childbearing in perpetuating an society.
Therein lies the problem. The concept of marriage (and everything having to do with "the birds and the bees") is something that parents ought to teach their children.
Whenever you get the government involved, it usually goes wrong.