Why is marriage a legal institution?

All this gay marriage talk got me thinking…
Why does the government have anything to do with marriage? I dont understand why the government needs to “solidify” a relationship. Why does one have to ask the government permission to get married?
Gays are fighting for the government to give them the same rights as straight people. Don’t our inalienable rights come from our creator, not the government? Doesn’t the fact that the government issues a license for marriage, make it a privilege not a right in this sense?
Shouldn’t we start shifting the conversation to all people(except corporations) being treated as individuals by the government and return marriage and partnerships to their religious and cultural roots?