Quote Originally Posted by Fynlar View Post

It doesn't matter what way you spin it, here. It is not correct because they are not the same things.
According to the Merriam's dictionary and a thesaurus they basically are.

In the dictionary "gender" means "the state of being male or female" and under synonyms "sex" is the first one listed. The first synonymy for "sex" is "gender" not all that surprising right? Just in case anyone is confused "synonym" means "the same or nearly the same meaning". A thesaurus and the Merriam's dictionary are both extremely credible sources for definitions for the American English language.

Tl;dr "sex" and "gender" are interchangeable.

just about every trans person in existence serves as proof
That's not credible (or proof at all) proof that sex and gender are completely different.