Mark Steyn has expressed how I feel about gay marriage far better than I could.
One of several money quotes:
What gives marriage its unique, immutable character is its role as an organizing principle of western society. What happens when you tinker with that organizing principle – with instant divorce, common-law relationships, “partners”? Well, in Europe, as Brooks has noticed in another column, you wind up with 1.4 children per couple. Italian women whose mothers had six kids by the age of 23 now have one designer child at 39. In other words, the state’s interest in this question is as basic as it gets: on the evidence of the last 40 years, when society experiments with traditional family models, it winds up with no children – and thus, as Europe and Japan are discovering, no future.
There's more than just individual sexual preferences at stake here.