OK but never said it wasn't, the problem arrises when the government starts passing out goodies BECAUSE of Marriage. Better tax rates, Social Security Benefits etc...
Once that was in play then the "equal under the law" aspect is in play.
Nobody is banned from marriage because of their sexual proclivities. Everybody can marry as long as you marry a member of the opposite sex that meets the age and familial requirements. That is the definition of equal under the law.
Back when slavery was still being practiced in this hemisphere one of the marks of the slave was their inability to enjoy the benefits of the civil marriage contract. The slaveholders resisted any notion of them having access to that, in the knowledge that many of the most important benefits of liberty are associated with it. Inheritance rights. Ownership of real property. Parental rights. And a whole lot more.
Those who won’t fiercely defend the institution of marriage in this country, at every level and on every front, are, whether they know it or not, helping in the drive to reduce this free people to the position of slaves.