Where do you draw the line? Why not provide a legally sanctioned "marital" association between people and pets, or adults and children, or groups of people. How about prostitution - should that be made legal in all states? Why not 1 hour "marriages".
Your argument that gay marriage ought to be legal because gay unions exists is specious. Laws exist to inhibit socially harmful behaviors that people engage in.
Gay sex is socially and individually harmful. That has been known to cultures for many thousands of years -- the knowledge codified as "sinful" behavior. And those who do not stand against sinful behavior, as immoral. How come we are so smart that we can discard this long legacy from our forbearers?
Personally, I'd draw the line before letting adults marry children or animals or multiple partners. But look, we could go around and around on this forever and I won't convince you to change your mind and you won't convince me. I'm tired of your insults and I'm not going to waste another second arguing with you. Let's just agree to disagree and be done with it.