Here's a clue, it doesn't need to be. Do you know why? Because the 14th Amendment mandates states to provide all persons within their jurisdiction equal protection of the laws.
Can you indicate a compelling state interest served by limiting the legal protections of man to being between a man and a woman?
If you can't, then such a restriction cannot pass the judicial level of review of strict scrutiny, and the restriction is unconstitutional.
If the state redefines the ages-old definition of marriage, under the guise of "equality under the law," then no other proposed form of "marriage" could be denied to any citizen.