As near as I can tell, there's two ways to look at marriage. First, as a civil institution. Second, as a religious institution.
If you take marriage as a religious institution, the idea of the Government having authority to define what is, and isn't acceptable in the practice of your faith should fill you with horror and dread. The idea that legally, the Westboro Baptist "Church" is less of a threat to the moral well-being of you and your children than two people promising to love each other is insane. The very reason we allow the first, the idea that no matter how much you dislike someone else's ideas, it's not the government's job to stop them from expressing them, is why the Bill of Rights exists. Not just the 1st amendment, but all of them. If you are religious, even if you think that homosexuality is a sin, even if you think that a same-sex marriage isn't valid, you should be afraid of the government restricting any expression of faith.
If marriage is a civil construct, then there's no valid argument against same-sex marriage (or polygamy if you want to be honest with yourself.) From a governmental standpoint, marriage is a specialized branch of contract law that deals with shared property and liability and providing for the welfare of children. Regardless of if the ceremony takes place in a church, a courthouse or doesn't happen at all (in the case of common-law marriages), the government treats married as married. At no point in filling out your 1040 does it ask if you love your spouse. No where on the forms to assign a beneficiary does it ask if you asked your husband or wife how they're feeling lately. When the hospital staff decides who can see you when your sick, they don't ask when the last time you said "I love you." was. From a legal perspective, it's a binary state. You sign the license and complete the paperwork and you're married. Or you don't, and you're not.
And if you believe that someone else's marriage is a threat to yours, you need to look at your own relationship and ask yourself why you'd allow any external force to have an influence on what you stood up before your family and friends and pledged was unbreakable.