My "case" was we would see the end of the marital tax incentives. Wasn't so much commenting on your other points.
But now that you made them...what you are not looking at is the marginalization of marriage in general. People staying together in one household has many societal and economic benefits to society - I am sure you would not argue that. So he government provides a financial incentive and other legal rights to encourage people to do so. If/when marriage as a legal status is removed - then people will naturally become more nomadic and abandon each other which will have the opposite effect.
My concern is not so much gay marriage, which I don't really have an issue with - but the continuance of the erosion of marriage in our society as a whole. If anybody and everybody can "get married" - then the value is lost.
We will also soon see "couple marriages"...in which one couple can marry another couple.
Marriage has existed for thousands of years. Are you suggesting that if the gov't stops essentially
paying people to be married, the institution will evaporate?
Why not give tax breaks to families with children on the honor roll? Why not give tax breaks to people who stay employed? To people who perform civil services?
Why not impose extra taxes on those who spend unwisely, or gamble, or eat too much red meat? Why not add more taxes to those who are on Wall Street for their effects on the economy?
Taxation is to provide funding for the gov't, nothing more. To use taxation as a form of social engineering or to somehow promote or reward morality is wrong on so many levels.