Marriage is a reality, an organic institution, it is a tradition and a norm, for the majority of individuals, no conceptualisation is required and its practice is restricted to men + woman.
There's no equivalence what so ever. Its a little like proclaiming that two homosexual men have the right to have vaginal sex with one another. Its obvious they can not have vaginal sex with one another, they are both men do not possess vaginas.
Its still not clear to me why homosexuals and their support want to remove to try and transform marriage into something which robs it of the meaning which the majority attach to it, I really dont see the benefit to homosexuals, is it going to make homosexuality more acceptable to those who do not consider it acceptable? No. Will it void any rejection which individual homosexuals have experienced by having homosexuality apparently endorsed by the state? No. Will it mean that religious and other perceived "foes" of homosexuality will offer ceremonies including same sex "marriages"? No. Will it make homosexuality more widespread than heterosexuality? That's an open question but also not the reason which most homosexuals attach to wanting reforms such as the transformation of marriage from meaning what it always has to meaning whatever the state says it does.