[dragonwriter]

> How do we know that?

Because marriage equality is a well-established term with a clear unambiguous usage in political discussions. Sort of like "pro-life" and "pro-choice", which could (divorced from context) each have a wide variety of possible meanings, but which, in the actual context of modern American political debate, have very specific meanings in terms of particular opposing positions regarding abortion policy.

> It's a marketing myth that "gay marriage" is the same thing as "marriage equality".

It would be more accurate to say that "marriage equality" is a brand that has been established by advocates of legal marriage without distinction based on the sex of the partners.