Robert Knight outlines it exactly right in the above titled column found here. An excerpt:
We hear a lot of people falsely comparing this to the fight for racial equality. But one’s skin color or ethnicity have no intrinsic moral content, so discriminating upon that basis was wrong then and it’s wrong now. But sexual behavior always has moral and social implications because it is volitional. Just because we have temptations to do something, that does not make it right.
Marriage is honored and protected in secular law only because it is indispensable to civilization. It gives us the best chance to continue the human race by producing well-adjusted children who can take up the responsibilities of the next generation.
Another good one – on the same topic – is this one by Michael Brown: “Gay Rights Still Trumping Freedom of Speech, Religion.”