There's good reason for the government to be in the marriage business. The civil marriage contract provides for the care and feeding of children. Unless you would prefer the courts to decide individual responsibilities for each parent on a case by case basis. Marriage also protects the rest of the citizenry from having to financialy provide for children not of their own making. Unfortunately the no-fault divorce laws that started in California in 1969 has caused a huge increase in single parent homes, deadbeat dads, and increased tax bills to take care of their children. Ditching marriage would only take more money out of my pocket to care for someone else's kids.
You've given good reasons for the institution of marriage, but you haven't given any good reasons for government to be involved in it. You have, however, given some dandy arguments in support of civil unions. Coming full circle to this discussion, civil unions have nothing whatever to do with whom you care to share your bed.