This is another example of people redefining words then claiming other people are unenlightened because they use the original meaning and not the redefinition.
Gender has been used as a synonym for sex for a long time. Merriam-Webster and The Free Dictionary list that meaning before the more recent meaning.
You're simply wrong to claim the way you use the word is the only way it can be used.
It could be useful to have a way to distinguish between the biological and social aspects of sex/gender, though. Because those are different things, so it kinda makes sense to have different words for it.
Telling people they're wrong to use a word in an old way is not very constructive, but language changes all the time, and when everybody starts using the words that way, then that's what they will mean.
