This, but I'd go a step further and say that any sort of union is none of the government's business.
Trust me, it is. Not only is it their business, but it's their business to enforce basic rights of equal partners. Generally, the state needs tobe involved to.
1) Grant next of kin rights to partner
2) Ensure tax/insurance breaks to partner
3) Respect visa/immigration rights to partner
4) Ensure partner is protected/supported financially in case of an end of the civil union
5) Ensure partner gets proper visitation rights to any offspring in case of divorce
6) Loads more
If you want to see a perfect example of how to completely shit the mattress on all of the above, simply look over to the world's 3rd largest economy, which is basically "crime free" (huge asterisk), toilets wipe your ass for you, and you can eat off the floor it's so clean: Japan. Terms to Google for some horrific reading. "Japan Parental Abduction". "Japan Divorce" (lol). "Japan Spousal Support". "Japan Dual Custody" <- that one doesn't exist by the way.
The state needs to be present for the same reason the state needs programs to take care of the poor: b/c people are fucktards, and a bottom-rung level needs to be set (and enforced) regarding how badly you're willing to let your citizens be treated. Hate to say it but people can't govern themselves.
There should be no tax benefits etc for married people.
Dependents. They cost money, and you should pay less taxes for more of those you have.
however you slice it, "marriage" is always going to have a religious tinge to it,
When the US government actually separates itself from the church, many moons from now, I'm sure the religious right will start bitching that they want a government which upholds their religion.
New PowerPuff Girls. They all have evil pornstart eyelashes.