And that denial is based on sex, not relationship of the people involved (incest) or number of people or number of contracts (polygamy). Sex/gender is at a middle-tier scrutiny level for equal protection, while both relationship of the people and number would be at the rational basis scrutiny level. Plus, there are reasonable defenses for the states to base not allowing both incest and polygamy based on either possible harm to at least one of the two involved or limitations in the contract itself or reasonable need to limit how many can be involved in the personal contract due to benefits offered.
If someone wishes to fight for either incest marriages or polygamy, they would need to do so on their own standing. I haven't seen a single person say that SSM should be legal just because interracial marriage was legalized. There are always other arguments with these and not a single shread of evidence that same sex marriages or relationships are harmful to either person involved or even society.