In the eyes of the law, marriage is just a legal contract that fails to incorporate love. People don't have the right to marry the one they love. Especially if the one they love doesn't want to marry them.
You might as well say that a law stipulating that 'no two people who both have a Christian name beginning with the letter C be allowed to marry' is not discriminatory.
Everyone is subject to the same law.
If your name happens to begin with a C you can either find someone whose name does not begin with a C or change your own name.
Completely fair, unbiased and equal to all. Obviously.
Seriously - How exactly is a law restricting two consenting adults from legal union based on sex any different in terms of rationale or principle from the example above?
Edited by Straggler, : No reason given.