Because civil unions don't give us the exact same rights. It's not stepping on anyone's moral toes and in fact is none of anyone's business except the couple getting married. Look at how the issue is used by Republican politicians; that tells you all you need to know about it.
Such bullshit. You have plenty of rights that don't require a marriage license, you don't need special rights and you sure as Hell don't need to run your lifestyle choice down the throats of civilized people.