The most basic problem with gay marriage, for all the non Torah observant here such as crisis and Charlie who seem to think that the Torah view is not important, is the redefinition of marriage.
Marriage was the beginning of a family unit, including all that is included in a family. It is not merely a financial contract, which is what I see some proposing here, including it as a reason why marriage in itself is not osur. Because of that were the case how can constructive abandonment be a reason for divorce? That has nothing do with finances and has everything to do with a marriage presupposing intimate relations.
The deconstruction of a family unit is something that should concern any reasonable person who is concerned about society in general. If liberals feel left out from this definition, that is their own fault.
It is pointless to expand further here, because the staunch liberals who post here seem less than intelligent and have proven concretely that they have issues with reading comprehension.