Whenever I see questions like this I remember this study: Video game experiment leads to insights into sexism .

TLDR is that it's a skill issue. Lower ranked male players are more likely to be misogynistic than higher ranking players. Read into that what you will.

As far as I’m concerned, not understanding this mentality is something to be proud of.

It seems like nonsense to even be wasting time arguing about whether she's a woman or not on a car forum, but how did it even come up?

Unfortunately, much of the car enthusiast community is filled with extremely bigoted and closed minded people. It’s a big problem.

If you want, feel free to tell your friend about the Superfast Matt Discord server (Matt is a popular car YouTuber, and has a very active car enthusiast community on his Discord). They have a strict zero tolerance policy for that kind of shit, and everyone there is very accepting of new people. She can find an invite at discord dot gg slash superfastmatt

(Not trying to advertise, just hoping to connect your friend with the kind of people she’s looking for!)

Same men obsessed with making a woman prove she's a woman to use a restroom. Just plain weird.

Guys can be strangely insecure about being "a man". If you tell yourself that you're a manly man because you're into cars which is a manly thing, then when a woman is also into that you either have to reject the uncomfortable idea or start asking yourself some more difficult questions about what being a man really means to you.

Although I'm a man I'm not an expert by any means but that's my read. Could be just plain simple misogyny as well.

My friend joined a car forum, because she’s into cars.

...and...

Apparently the way she talked was too “proper” to be “girly” and people responded that her knowledge of car components was a “red flag” that she must secretly be a man.

With this last comment right here it sounds like she is fairly knowledgeable about cars and the men on that community see "car knowledge" as their personal domain and expertise. If (gasp!) a woman knew more than a man on a car thing, then the man would be emasculated!! Since she was demonstrating knowledge, and perhaps more knowledge than some men, the men could only accept they were "beating" by another man and therefor they themselves wouldn't be emasculated.

tldr; the men in that community are terrified of being perceived as "less manly".

There are people trying to do genitalia checks in bathrooms in supermarkets. Americans are seriously fucked in the head right now.

Honestly, I think that one of the reasons for such scenarios may be that it's easier to try to demolish the idea of there being an actual woman present than accept that she has no romantic/whatever interest in them. And I don't think this is driven primarily by an ego impulse, but by hopelessness.

Of course, there are many other things which factor into such situations (a partially justified caution in the example you've described, for instance, as catfishing can end up being very painful for vulnerable people, even in "milder" forms), like societal norms ("gurls aren't supposed to like icky boy things!"), maybe some elitism, some group resistance, a sort of hazing, etc. But I still can't shake the feeling that even these stem from hopelessness in various ways.

I'm sorry your friend experienced this type of behaviour, and I'm sorry it did some splash damage on you as well... It is, indeed, very disappointing to see such behaviours still being not only prevalent, but even frequently reinforced as standard nowadays... It's a disappointingly hostile world in many places. But if I may point out the silver lining in this, at least nobody can take away your friend's passion!

Not trying to be sexist, but why does it matter shes a woman on this forum in the first place? It sucks, but there are places on the internet where just stating you're a woman gets you extra attention or help from desperate men. So maybe the forum is just tired of people coming on, stating their a woman whether true or not, and getting help way quicker than if no gender was to be said at all?

I know it paints me in a bad light but whatever, there is no reason to go on the internet and state your gender unless you're looking for extra attention. Your friend could have just asked the question and left out gender talk completely. There was no need to state she was a woman.

I know this a pretty sexist example, but viva la dirt league put out a gaming video explaining why some guys are over it.

All people on the Internet are men.

Do you know they're all men trying to prove this?

This is a rhetorical question, getting at the point that we don't know if you're a man, woman, or one eyed flying purple people eater, so even engaging in any thought about this is a waste of energy.

Best you can do is say the same thing - this is a waste of energy can we go back to talking about cars. Cause frankly I don't give a shit what your dangly bits are, it's irrelevant to this conversation.