It's not (primarily). It's about gay partners being given equal protection under the law as heterosexual partners: tax benefits, visitation rights, etc..

I wonder, though, if the polygamysts are really concerned about tax benefits, etc. There are probably aspects they're concerned about, but I'd bet if some time were taken and people didn't feel morally opposed to the notion of polyagmy (which is just a baseless assumption I'm going on) then something could be worked out where the polyagmy could be made legal. I guess my point is that polygamy is illegal not because of taxes, visitation, etc. but because in general people aren't comfortable with it.

It's just that religion wants to keep the "marriage" word to itself.

Which I think isn't unreasonable. But as someone will quickly point out if I don't say it, the question is whether marraige began as a religious instituation or is just a natural development. But we've hashed this particular issue to death, I'm just interested in how people feel about polyagmy.