This strikes fundmentally against the power of the people since the judiciary is the most removed from direct control by the people. We elect legislators to write the law, we elect a president to the executive to carry out the laws and lead, but the judiciary is the least democractic part of our government. Federal judges are not elected to their positions, and most state judges never face the ballot box either. This puts enormous power into their hands without adequite checks and balances and the threat becomes greater when they exercise not only their power to judge laws, but co-opt that of the legislature to write them as well.
Now, I was going to talk more specifically about gay marriage and civil unions, but I got a bit sidetracked as you've no doubt noticed. My problem with gay marriage is that it's changing the definition of marriage which annoys me. For many, marriage is also a religious institution and in fact began with the church. I simply don't see why the definition needs to be expanded when there are civil unions as an alternative. What's to stop someone from marrying their pet dog next? Homosexuals should have the same rights, privledges, and responsibilities as married heterosexuals but I simply can't agree with calling it marriage.