You may be interested in my promotion vs. prohibition argument, which I made in another thread:
http://www.freerepublic.com/focus/f-news/1032027/posts?page=497#497
You may not agree with the argument, of course :) But it illuminates my reasoning as to why it is at least legally acceptable for the government to restrict marriage to a man and a woman.
Also note that the government is not just "discriminating" against gay couples with its marriage policy, but also incestuous couples and polygamous unions as well.