That's a false analogy because it presumes that marriage between gays is even possible. Those with the strongest arguments against gay marriage hold that:

- marriage is a natural institution that the state recognizes (the state, technically, does not marry people)

- marriage is ultimately rooted in the intrinsic capacity (though not necessarily the successful exercise e.g. impotence does not render marriage invalid) for sexual reproduction and the complementarity of the sexes

Therefore, they hold that there simply is no such thing as gay marriage, and you cannot deny what does not exist.

Marriage is an abstraction invented by humans. This is why the details vary from culture to culture.

Humans didn't invent gravity. This is why it is the same in the US as it is in Asia, as well as everywhere else in the universe.

Thus, gay marriage can easily exist. People simply have to believe that it is a useful abstraction.