It boils down to the fear of having to suffer.. If our fears were to become realities, they would all cause some form of suffering--injury, pain, illness, starvation, exposure, homelessness, isolation, etc. And in my opinion it is having to live through this suffering that we dread the most.
km: "Calling same sex unions "contracts between adults" offends me." I agree with this part of your statement.
Wayfairer: "Five years from now men will marry children and women marry barn animals." That particular slippery slope fallacy is a category mistake. Children and barn animals could not by law be considered consenting adult humans.
People have been insisting that society will collapse when same sex marriage is legalized, for decades. It didn't, so now people insist God will smite our country for condoning sodomy. Well, that's not really a prediction without a timeline -- it's more of a waggling finger.
Every generation since before Socrates complains that the generation after their own is morally worse. But all these fears of scary, but very vague bugbears haven't materialize in other countries that are several years ahead of the US in legalizing SS marriage, and show no signs of happening here, either.