Gender is defined as: the state of being male or female (typically used with reference to social and cultural differences rather than biological ones).
For most people, your gender and your sex are congruent. For others, it isn't.
The definition of gender is not universally agreed upon. And it's pretty obvious that that definition is not the one the person you are replying to uses.
Definitions are also not fact, by definition.
There are plenty of valid arguments for calling trans people the gender they want to be. This isn't one since it isn't going to convince anyone who doesn't already agree with you.
Edit: The why do you care part is a valid argument, was addressing the rest of your comment
There are 7126 subscribers to the subreddit you linked. A notable feminist subreddit that is actively not transphobic (that I am not going to link to because I love it and I do not want it to be brigaded) has 214400 subscribers. So your sample is about 3% of the internet population if we believe that reddit is a representative sample.
I sincerely believe that the vast majority of women are tired of being used as an excuse for bigotry.
Absolutely not. The OP linked to reddit to demonstrate their viewpoint was shared by women, and I was countering their argument by demonstrating that it is relatively unpopular among women on the site that they linked.
I definitely understand the downvotes now that the comment has been flagged and is not able to be seen.
Nobody here has argued anything about biological sex. We're talking about personal identity, which is psychological and cultural instead of microbiological.
Gender is defined as: the state of being male or female (typically used with reference to social and cultural differences rather than biological ones).
For most people, your gender and your sex are congruent. For others, it isn't.
Even if you don't agree: Why do you care?