There are two issues people often discuss regarding same sex marriage.
The first issue is the constitutional issue, and is meaningless to most people who don’t know the 14th Amendment from their left nostril.
The second issue is whether we want to restrict marriage to opposite sex people.
Now, looking at the historic definition of marriage really speaks more to the constitutional issue, and is discussed at length in the briefs for the CA Prop 8 case.
We should more be asking the second issue, which is why as society we don’t want same sex marriage.
For me, the answer is that I think homosexual acts are immoral, and I don’t want to see society endorsing it as a positive thing.
I think it is worse to endorse it by recognizing institutions based on it, than to simply not criminalize it.