Personally, I do link your genitals to your gender, but I did absolutely did NOT want to assault your sense of self! In fact, it's because I respect other's feelings I asked before just making such a thread. It was the core value I was raised with to be polite and mindful of others.
Ok, so this is not an opinion, this is just what it is. But if you study advanced biology gender and sex are considered linked as for most people they are in alignment, but NOT the same. Basics,
Sex: The biology that makes up the body, this is extremely complicated and in lower biology they teach of the two most common chromone patterns XX and XY. But it is far more complicated than that not only are there XXY, X, YYX etc. but there are far more characteristics that make the sex of the body.
Gender: A social construct of justifiable and acceptable actions that displays a persons gender identity, the things that define what is and is not acceptable within the gender expression can and douse change based upon time and location.
So put simply, sex is what you are, gender is what you present to the world. In the most stereotypical way that would be boys: Blue, trucks, trains, science Girls: Pink, baby's, dolls, cooking, cleaning. Now not everyone will fit the stereotype, nor should they and that is why there are a lot of things that associate to gender. And as I said those attributes change based on time and location. In the 1800 it would be men wearing make up, showing emotions and wearing high heels. In the middle east a woman would be expected to fully cover vs in America a woman can wear far less.
No I am a gender abolitionist so I don't believe the category's offer utility. I believe there is no reason a woman can not compete against a man in anything if she wants to and vise versa and if separating the sex's no longer matters gender offers no utility.
So there is a difference between the two and regardless of my wishes we live in a world where women are expected to fit in the female gender stereotype and men in the male. but nothing is that simple an plenty of people can and do break out of that expressing their gender identity in new and beautiful ways. Hopefully that helps to understand why they is an association between gender and sex but why the two are not the same.
I am grateful you asked but lets run a hypothetical, the same one that has been done with the whole bathroom thing. Lets say there is a thread like that made, how do you police it? Because you would need some way to ensure it is only people with vaginas joining. Are we going to have sending your photo to some random place to prove your genitals? If so, who do we trust with that? and if it happens how many are going to be comfortable doing that? I know I wouldn't.
Lets now run the inclusive version. Person joins and says I am a girl, thread says ok, you are a girl. Person is treated as a girl. The end. Will there be some people that are uncomfortable, sure. But usually the reasons they are uncomfortable with it are form a place of intolerance, disgust or ignorance. None of these justify exclusion really.
Preference is acceptable as a reason when talking about creating a relationship of some kind, then there is reasons to have specifics in what you want and you will find that in the personals section. But in the threads where we all interact, there is no lasting bound and as I said, you can politely refuse to do a task. But it seems that the reason is 'trans people with penis, ewww icky' and I am sorry but that is reductive and discriminatory considering it is in relation to a thread game.