Why should the state even care?

It seems to me that the original purpose of marriage was to support forming families and raising children who will be the next generation of society. The state, being (theoretically) the collective will of The People in a supposedly representative system, has a legitimate interest in the future of the society. This suggests that perhaps the notion of "civil union" should be further unbundled, but the first step is still getting the state out of the marriage business.