This really has nothing to do with sexual orientation.

It's not gay marriage, it's same-sex marriage. This is a subtle but extremely important distinction.

Gay people have always been allowed to marry. They haven't been allowed to marry the people they want, but a gay man was allowed to marry a gay woman in every state in the union. Similarly, straight people were not allowed to marry other straight people of the same sex.

Marriage is just a matter of straight-up sex discrimination. Bob can marry Jane but Susan can't, because Bob is a man and Susan is a woman. That's clearly discrimination based purely on the sex of the participants (and not the sexual orientation, which is merely the thing that might cause Susan to want to do this, but not relevant to whether it's allowed) and IMO a clear violation of the equal protection clause.

Laws are supposed to be sex blind. If a man can legally do something, a woman should be able to legally do it as well. That was not the case with marriage before this decision.

I should have phrased my earlier comment more specific to same-sex marriage, that's true. I don't think your second argument that marriage is sex discrimination is valid though. As you point out, both men and women have been allowed to enter into non same-sex marriages, regardless of their orientation or gender.

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.

I am not allowed to marry Steve, purely because I am a man. If I were a woman, I would be allowed. How is that not sex discrimination?

Because the institution, historically speaking, has been between people of different genders. It is only recently that people have even considered allowing same-sex marriages. It certainly wasn't considered or was part of the intent of the 14th amendment, when it was added to the constitution.

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.

Why does the history of the thing have any bearing on whether something is sex discrimination?

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 mostly meant within the context of the current law and the historical understanding that marriage was only between a man and a woman when the 14th amendment was added. In my opinion, the constitution, as it currently stands, doesn't consider prohibiting same-sex marriage to be gender-based discrimination.

Seems to me that the question of whether denying same-sex marriage is sex discrimination lies outside of law, and the law's purpose is simply to discuss which forms of discrimination are legal. I would say that at the time the 14th Amendment was written, it would have assumed that sex discrimination in the context of who can get married was allowed, not that it didn't consider it to be discrimination in the first place. But that's really just splitting hairs.

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.

That is correct. Change in attitude can be reflected by due legislative process, but some of the liberal victories always believed in Fiat, either from the POTUS or SCOTUS.