For all the things we could call "evil"... honestly, this is pretty low on the list.
I mean, I call basically every major mistake that the government does evil, but calling any stance on gay marriage (That doesn't involve physical violence against homosexuals, anyway) "evil" seems like a stretch.
All things considered, it is pretty low on the list of evils. But that doesn't mean it's not still evil.
Imagine you and the girl you love. You're enjoying a supreme moment of bliss, in a loving embrace. Time seems to stand still, and it occurs to you this must be what eternity feels like as you realize you can't figure out if it's been a minute, or an hour, that you've been lost in this feeling of amazing love and oneness with her.
The two of you decide to commit to be together forever. So you go get married. But the problem is, there's a law saying the two of you cannot get married. So your marriage isn't really legal. It's just symbolic. You decide it doesn't matter, because it's just a piece of paper anyway, and you don't need anyone's permission, especially not the government's. So you're happily married.
Then eighteen years later, in a tragic accident one day, your life comes to an abrupt end. Unfortunately, you never wrote out a will. And since you earned all the money and your wife had no credit history, your name is on everything and hers is on nothing except the checking account. (I know, I know, just bear with me here. Lots of people don't write out a will or establish a trust while there's still time, as stupid as it is.)
So you've bit the dust, and now the product of all your hard work--the home you've paid for, the cars, all the money in the savings account and 401(k), everything--it's all being handed over to your mother and father who disowned you 18 years ago. They disowned you because you are a sick twisted abomination who is going to burn in hell for eternity because you thumbed your nose at God and committed blasphemy against their religious beliefs--the beliefs that said it was wrong for you to fall in love with that girl and/or marry her.
Now your "wife," the love of your life, files a lawsuit claiming she is entitled to the house, the car, the bank accounts, everything. After all, she's been with you, been your loving partner, for the last 18 years. And you loved her more than life itself.
But the courts say to your widow, "That doesn't count for anything. You're not his wife, and he wasn't your husband. You were never his wife. You're not a relative. You were nothing more than a roommate, and one who didn't pay any rent at that. You are no one. You have no claim to any of this property. It belongs to the next of kin and that isn't you."
So your parents who called you vile names and screamed at you the last time you saw or spoke to them 18 years ago, the parents who screamed with hatred that you're not their son anymore, and in their eyes you no longer exist, and told you that they'll be happy when your soul is being eaten by maggots in hell for eternity for turning your back on God the way you did... they take all your hard earned property and your wife gets nothing. She is soon evicted from the loving home the two of you shared together and now she has to figure out where to go and what to do. All because it never occurred to you that you *didn't* have plenty of time left to write up a will, and because the law does not recognize your marriage.
Now... In this situation, can you see where some people would say that laws restricting the freedom of people to marry whomever they choose are evil? I don't think it's that much of a stretch. (By the way, the above is based on a very true story. Only it was two guys, not a man and a woman.)