The OP knows that gay marriage is a heated political issue. I think it's a fair inference that they know polygamy is, too. The OP brings up both, as well as the question of whether "polies [should] campaign for the right for a civil union."

I'm pretty sure that the OP is talking about polyamory, not polygamy (I don't know if you were unaware of that, or if you deliberately brought up polygamy as an analogy).

If I wanted to be cynical I'd say that polygamy and polyamory describe pretty much the same phenomenon, except that polygamy is detestable and reactionary and oppressive, whereas polyamory is the complete opposite; or that polyamory is when it's done by fashionable white people, and polygamy is when it's done by weird brown foreigners (I don't think either of these is a fair statement!).

I agree with your main point - I don't particularly want to see more discussions of social policy on LessWrong, especially when they don't push the analysis very far.