I take somewhat of a libertarian view, get governement out.
However, if government is in, then it would seem to be the obligation of those who want to exclude others to make their case, as opposed to the other way around.
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.
The law, precedent, and tradition all agree on what individuals are eligible for marriage or any legal equivalent. Those who want to change it have the greater obligation to state persuasively why the change is necessary.
But naturally it is also fair to ask why things are the way they are. That is very straightforward. I repeat my statistics:
Number of people on this planet born of heterosexual unions: billions and billions
Number of people born of homosexual unions: zero.
The idea that we are somehow beyond the need for the state to involve itself in the regulation of procreation is laughable. Marriage is the institution around which this regulation centers.
To stop excluding gay couples from the legal structures of marriage is tantamount to stating that, as a matter of principle, procreation is irrelevant to marriage.