I am all for civil unions and I don't have a problem with what people do inside or outside the bedroom, but devaluing marriage does impact me.
Firstly, it impacts you because you choose to let it impact you emotionally. Allowing someone else to get married has no direct effect on your life. Gay marriage laws in no way impact your ability to get married or any of your other rights. Laws are crafted to protect people's rights, not to stop them from being upset because they don't feel comfortable with what someone else is doing.
Secondly, the burden of proof is on you to show that allowing gays to marry in anyway devalues marriage. If you're really worried about the sanctity of marriage, you'd should be focusing on quickie marriages, serial marriages, and divorce rates, all of which have been trending up since long before any state passed gay marriage. If there's a threat to marriage, it's that people aren't interested anymore, or else don't take the commitment seriously. How is more people wanting to participate a threat to the institution?
It boggles my mind that you can't draw parallels between your own biracial marriage and gay marriage issues. Forty or fifty years ago, a biracial marriage was at least as taboo as a gay marriage is today, and until 1967, was illegal throughout much of the US, including many of the same states that are now trying to prevent gay marriage. Back then, people made the same arguments: biracial marriage was a threat to society, a threat to family values, a threat to the institution of marriage. Did you marry a black woman because you were trying to overturn marriage? Of course not. Open your brain for half a second, and try to imagine that gay couples today are going through the exact same thing. They are being targeted, harassed, and vilified simply for being in love.
And if nothing else, just recognize that you're on the wrong damn side of history. In thirty of forty more years, gay marriage will be legal in every state and will be widely accepted, just as biracial marriage is today. And the children of that age will look back and think how ignorant and how bigoted it was to try to block that.