A friend responded to the previous post so I thought I'd paste my reply here:
But my argument is the State will set an example EITHER way. If the state is neutral towards all sexual relationships, the example it will set is that there is nothing better or worse about any of them. This will make it easier for people to get into the wrong types of relationships. Since the state will set an example EITHER way, then it might as well set the example or standard at traditional marriage.
If the message from the state is different from the Church's, then it will only make it more, not less, difficult for the Church to instruct her flock.
You're right that we've been designed for trad marriage and not gay marriage, but that doesn't mean our design is an incentive. Our knowledge of our design is implicit and is suppressed by sin. It is up to the institutions of our society (family, church, and state) to make explicit what is implicit. To bring to light what is in the dark. And it is important that all the institutions are on the the same page. Differing messages or examples will only call into question the validity of the authentic one. I say the authentic one b/c the false one will be easier to believe b/c it is, well, easier.