If you want to add a law to consider prohibiting same-sex marriage as discriminatory, then go through the process to add a law through the legislative process. Changing the meaning and intent of the current laws through the judicial process seems like the wrong way to go about it. It just isn't currently part of the constitution.
It seems quite obvious to me: if a man is legally permitted to do X, and a woman is not, purely because she's a woman, then that's sex discrimination. It doesn't matter what X is or how much history there is around it.
If you want to argue that the 14th Amendment was not intended to prevent sex discrimination in marriage, I'm right there with you. But I don't see how you can argue that requiring marriage to be one man and one woman is not sex discrimination.
I think the important bits are: the 14th Amendment wouldn't have been considered to allow this when it was written, but now it is.
What's in question here is whether its discriminatory to not allow same-sex marriages to occur and whether states should be able to determine what marriage means and restrict it accordingly. Marriage has traditionally been between a man and a woman and certainly wasn't intended to be part of the 14th amendment when it was added to the constitution in 1868.
To add it more than 100 years later seems like a case of the judicial branch legislating and adding to the law, more than interpreting existing law in the context and intent of which it was passed.