da5id: Is it? What about the right of a man to marry his sister, or cousin? Or to marry someone under age? or to marry more than one person? or to marry his dog? 'Discrimination' does not always mean 'bad' and the State rightly discriminates against certain types of marriage, including (at the moment) gay marriage.
You can't marry your sister or your cousin because sexual relations (An element common to almost all marriage-like relationships) can cause physical harm (problems with biological offspring as a result of inbreeding). You can't marry a child (or your dog) because they are not legally able to provide consent. The issue here is with the relationship itself, marriage is a flow on.
In the case of same-sex relationships, you can only use the above argument if you fundamentally this homosexual relationships are wrong and should be illegal - in which case you are not only fighting the wrong law, you are on the wrong side of history.