The government is legally obligated to give everyone the same rights, regardless of sex. If I, a man, am allowed to marry a woman, but Chloe, a woman, is not, then we clearly do not have the same rights. To bring us to equality, there are two options: either both of us are allowed to marry women, or neither of us are.
Something tells me that anyone who ran on a platform of eliminating legal marriage would be lynched for, you know, attacking marriage.