Lets starts by defining marriage. Marriage started out as and will continue to be a "religious sacrament", subsequently involving the church in political activities. "Separation of Church and State" pundits have a real problem here. If the state governs marriage licenses, they are literally bringing the church, and its moral standings and practices, into the middle of government actions.
Actually, this is not true. I think most protestants would argue
strongly against this idea, because it implies that marriage
was created by the Church, mediated by the Church, and therefore
could be tied to one denomination. "Only a marriage performed
by our church is valid"

Someone marries outside my denomination, I can declare it invalid.
We have enough trouble with people looking for ways to duck out of
their marriages. I don't want preachers further undermining peoples'
relationships by telling them that by converting to this or that
denomination you can consider your marriage null and void.

Society at large has a stake in this, because of the resources they
have to spend dealing with domestic violence, runaway children
and so on. That's your tax money.

Switching to civil unions for everyone is only a change in name.
Marriage is so weak an institution already that it can't be enforced.
Those wanting to cheat can't really be forced to meet their
obligations as it is.

Gay marriage is really about money. Taxes play a part, but the biggest
issue is health insurance. In most employer-provided health policies,
you save a bundle by covering your spouse and children. It saves the
government the burden of caring for those kids, and is an incentive
to get and remain married.

If gay people, who are essentially single, can cash in on this benefit,
the insurance companies may crack under the burden and withdraw
spousal benefits altogether. There is no such thing as a free lunch.