The simple answer is that I don't actually know why God determined that homosexual sex is a sin. He just did. Obviously, as a Christian, I'm going to follow what God says, and encourage other people to do so, regardless of how much it actually makes sense to me.
As for why I THINK its not allowed, I believe God designed sex for a very specific purpose, and he created male and female to come together in that role. I believe that the sex act was naturally created by God to exist in a marital relationship (I should point out that if its NOT in a marital relationship, its still wrong even if the genders are opposite.)
I believe God never designed sex to be between two people of the same gender, and in fact believe such a thing is a perversion of what God initially intended for sex to be.
As for the state, I certainly believe it should not enforce my views on sex. However, I am not going to support the state actually and actively endorsing a lifestyle I disagree with.
Now, the difference between a civil union seems miniscule on the surface, especially if its a civil union no matter the gender or a marriage no matter the gender, but the difference is very real. Civil Union simply states, and limits, the benefits to the legal realm. It makes no moral judgment as to the social or spiritual validity of the union. Marriage does. This point happens to be very important to me. Almost like how I'm OK with certain drugs being legal, but would never want to see the government encourage people to use drugs.