Why should same sex marriage be illegal in the U.S.?

With all due respect to the previous response, I have a different take on this issue.  The question assumes that same-sex marriage should be illegal in the United States, and that suggests a questioner who has already made a decision. 

First, we need to example what we mean when we say "illegal in the United States."  Presently, the United States Constitution does not allow the federal government to make same-sex marriage illegal. So the question posed also assumes a possibility that is non-existent. Legislation on marriage is left exclusively to each state, and as you are aware, some states have chosen to make same-sex marriage legal.  In order for the United States to make same-sex marriage illegal, there would have to be a Constitutional amendment proposed and passed.  This is exceedingly difficult to do, and I have provided a link that explains the process. 

Now, while I agree that marriage is a social construct, it is an odd hybrid today in that it is a social, legal, and religious construct.  In the United States, one can be married as a civil matter, a marriage sanctioned by the state, and never have a religious ceremony at all.  Some people do one or the other, while some people do both.  Given this situation, people who are opposed to same-sex marriage can certainly prevent these from being performed within their own religious institutions, which are exclusively private.  Since most of the opposition to same-sex marriage is religiously based, and since the First Amendment prevents the government from interfering with the practice of one's religion, whatever the religious reasons are, they must be respected within the confines of the religion.  The government can never force any religious institution to perform same-sex marriages, and that is as it should be.  People's religious beliefs must be respected and upheld in the United States. But that is not quite the same thing as allowing people's religious beliefs to dominate state matters. 

I must say that I do not understand what any of this has to do with the state allowing same-sex marriages.  Traditionally, society allowed slavery, child labor, domestic abuse, and any number of other social constructs that are now seen as abominable.  So, to argue that a social construct should continue because it has existed is not a compelling to me.