You're missing the main point: it's about discrimination.
A law that would forbid wearing a burqa as a societal trade-off would apply to everyone equally hence it wouldn't be discrimination. Even though it would affect the most people of a certain religion, it would also apply to secular burqa wearers.
The main issue with a law that forbids gays to marry, and the reason it's discrimination, is that it denies them a right that everyone else has.
Whether polygamy is legal or not has nothing to do with it. The law forbids a specific behavior (polygamy) and it applies to everyone so is not discriminatory. Constitutionally, it's in the same league as a law that forbids smoking in public places or, for that matter, killing people.
So you're left with only one potential argument: that gay marriage causes specific, societal harm that is not caused by non-gay marriage. I would like to see a solid proof of that. "It may have" is not one.
A law that would forbid wearing a burqa as a societal trade-off would apply to everyone equally hence it wouldn't be discrimination. Even though it would affect the most people of a certain religion, it would also apply to secular burqa wearers.
The main issue with a law that forbids gays to marry, and the reason it's discrimination, is that it denies them a right that everyone else has.
Whether polygamy is legal or not has nothing to do with it. The law forbids a specific behavior (polygamy) and it applies to everyone so is not discriminatory. Constitutionally, it's in the same league as a law that forbids smoking in public places or, for that matter, killing people.
So you're left with only one potential argument: that gay marriage causes specific, societal harm that is not caused by non-gay marriage. I would like to see a solid proof of that. "It may have" is not one.