First of all, religious arguments against or for anything should be irrelevant in civil arguments, it's a question of human rights. For example, the recent discussions on wearing burqas does center on whether their use is religiously dictated or not but rather the trade-offs of the rights between wearers and society at large.
This trade-off point is my main counter argument. Specifically, this means that gay marriage can have drawbacks for the society, e.g. (i) as others point out, the same logic (consenting adults, personal rights, etc.) can be used to argue for polygamy and other forms of relationships that are outside current society's norms; (ii) it may have adverse affects on the couples children.
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.
This trade-off point is my main counter argument. Specifically, this means that gay marriage can have drawbacks for the society, e.g. (i) as others point out, the same logic (consenting adults, personal rights, etc.) can be used to argue for polygamy and other forms of relationships that are outside current society's norms; (ii) it may have adverse affects on the couples children.