Would you accept the state declaring state involvement in marriage a violation of the establishment clause?
No.
Better than having civil unions for everyone, the government should simply separate itself completely from the institution of marriage. The government give official recognition to "marriages," "civil unions," or anything else. Instead, the institution should be completely private.
I don't want to change the entire concept of marriage, I just want to include same-sex couples. Polygamy and other forms would still be off limits.
That seems like an extremely arbitrary distinction. Why should polygamy be "off limits," but not same-sex marriage?