But to use your analogy, imagine for a moment that the FDA required all beef - even that mixed with pork or chicken products - to be marked as kosher.
"Kosher" is a term specific to Jewish dietary laws, while "marriage" is not specific to abrahamic religions. Furthermore, marking everything kosher would make it difficult to avoid pork, while calling gay marriage marriage makes it no more difficult for you to avoid marrying someone of your gender than it already was. Your analogy is flawed to the point of irrelevance.
Or, to put it another way: selling something as "kosher" when it's not is false advertising and falls under (secular) consumer protection laws, while calling gay marriage marriage does not, since no one's trying to sell you something.
Christians, Jews, and Muslims consider marriage an act of God, not of mankind. Surely you've heard the term, "A match made in Heaven", which alludes to God's involvement in bringing a man and a woman together. To call two men or two women married profanes a relationship considered sacred, and reduces marriage to mean, more or less, that two people are simply fucking each other on a permanent basis.
To call some relationship a marriage in no way affects any other relationship. How could it, if they are "acts of God" and "matches made in Heaven"?
It reduces the societal esteem of all marriages, because marriage no longer means that something sacred and wonderful has occurred between a man and a woman, but only that two people have decided to live with one another.
So basically, what this really comes down to is that high school never ends?
Also, it occurs to me that judging the same thing to be either sacred or profane based solely on the shape of the genitals of the people involved seems very much like something a mortal meatbag, rather than an eternal spirit, would do.
To further put this in perspective, consider how offensive it would be, if a police officer were to inquire about your wife, "Is that your bitch, or is she someone else's ho?" If it would be offensive to regard your relationship with your wife as nothing more meaningful than that of a prostitute, how much more offensive is it that the Supreme Court of the United States considers your marriage to be roughly analagous to two men committing sodomy.
Except, of course, no one has claimed that people having sex is the same as people getting married. Beyond that, you're a human being. Gays are human beings. Should we declare that gays are not, in fact, human beings, since otherwise someone might think that you are roughly analogous to them?