I know this is a matter of logic, what I am about to write, but, well.
Once you unhinge marriage from its constitutive gender opposition, so that it is no longer about a male and female, you effectively make gender irrelevant to it. Marriage, that most gendered of institutions, becomes gender-free.
And once that happens, it is essentially a genderless contract between adults.
So, in addition to making it indefensible to limiting it to two such adults, what reason is there to prevent any adults from making such a genderless contract. Why, indeed, should you not be able to marry your brother, or your uncle?
The fear of genetic damage only exists when there is conception. Without that, no barrier. And even if a man wants to marry his sister, a genetically damaged child can be aborted. No problem.
And how do you annul a genderless marriage? What, in the state's eyes, constitutes the sexual act that makes annulment impossible?