They have a website though, now.
Their points, in order.
Natural marriage is the foundation of a civilised society.
My tendency here is to simply add the suffix  to the text. The foundation of our society, last I checked, was freedom. Freedom of the speech, freedom of the press, freedom of assembly ... that sort of thing. The Taliban have marriage. The Vikings had marriage. Soviet Russia had marriage. Marriage - whether forced, voluntary or some combination thereof - has been around for a long time and I don't know of any correlation between its prevalence and level of civilization a society has.
Same-sex unions, as pseudo-marriages, involve gravely sinful acts that are never defensible, and the very assimilation to marriage damages marriage itself.
We're just going to ignore that "pseudo-marriage" phrase, as you're assuming your conclusion here. (i.e. see above, ). Gravely sinful acts? I direct you to read your bible for all of the other "gravely sinful" acts. Like eating lobster, crab, oysters or clams. Or weaving two kinds of fabric together. Or sewing two different crops in the same field. Or accidentally seeing your father naked or your wife while she's menstruating.
Also, I think you're using the word "assimilation" incorrectly. Good try, though. Big words can sometimes be impressive.
The law is a great teacher, and it encourages or discourages behaviour.
True that. You're 1 for 3, quite respectable in major league baseball. The law should serve as a guideline for ethics by, for example, demonstrating that everyone has equal rights and that one group of people can't force their morality on another unwilling group.
Government-backed same sex civil unions would encourage and normalise homosexual behaviour, and it would harm natural marriage, children, adults, and homosexuals themselves.
That's a lot of claiming you're doing there.
Yes, it would normalize homosexual behaviour. Why is that bad? What we've seen, with celibate priests for instance, is that making sex forbidden can cause some pretty harsh overreactions and perversions. I'd rather have gay people feel normal, not just for those reasons, but because there's really no reason for people who engage in consensual sex to feel bad about it.
Also, I'm living in Canada. We've had Evil, Gay Sex Marriages for years now and there's no evidence that it's destroyed or harmed Holy, Opposite Sex Marriages. Perhaps you could provide evidence.
And yes, the adopted kids of gay people turn out just fine.
The law should promote behaviours that are beneficial and prohibit (or at least not endorse) those that are destructive.
Did you read the link above? The study is pretty clear: "...the vast majority of studies show that children living with two mothers and children living with a mother and father have the same levels and qualities of social competence."
You'll have to explain how the Evil, Gay Scourge is destructive.
Therefore, the law should promote natural marriage, and it should provide no option for government-backed same-sex marriage or civil unions.
Therefore, nothing. None of your points above were accurate depictions of reality. Therefore, your conclusion is unsupported.
Meanwhile, take off that shirt. I believe you're wearing a shirt made of two types of fabric woven together and - while your god is explicitly in favour of bigotry and brutal punishments in that vein - he really doesn't like clothing made of two fabrics woven together.