What is wrong with saying not all ____ do ____?

It's been weighing on me since a few days ago, ao I'd really like to know so I can ammend my thoughts. There is a TL;DR at the bottom but pls read the paragraph after this for a but of context.

Someone jokingly said they wanted to oppress all "cishets" and make heterophobia real with a petition. This is cus some dumbass on tik tok wrote a bunch of transphobic (rather transmisia, they arent fearful or angry cus of fear) shit.

I know it was a joke, but i still said, "nah, im cis so im good. Dont lump us all together [with this idiot tiktoker], thanks." It wasnt that exact wording, but i declined and said not to make one idiot the face of a group and make it a stereotype.

Well some person, who I wasnt even talking to nor have i seen before said, "I'm cis too, read the room jesus christ. I bet youre one of the "not all men" mfs", and then another person who i never seen before also said, "right stfu with that "not all cis" bullshit.

First, I want to know how im suppoed to know if you're cis or not by looking at some reply you made. I need to know your superpower so I dont offend anyone else.

I replied to the first person (not OP) who replied to me and I told em that yeah, I was. I also added that I'm also:
>> Not all men are ______ (if i have to choose a word now, it'd probably be abusers)

>> Not all women are _______ (if i had to choose a word now, it'd probably be sluts or looking for sex, definitely not all.)

>>Not all [people who are bisexual]* cheat (because that is somehow a misconception on tik tok that all bisexual people HAVE to cheat which is BULLSHIT.)

>> not all [people whe are homosexual]* are lusting after you (ie. Just meeting eyes means they wanna do you. Like no, thats wrong).

>> not all [insert group] are doing [harmful, divisive stereotype].

I also said:
"Im all for taking down stereotypes, but perhaps stop imposing them on groups that these fucktards on tik tok belong to. I would agree to this post [ie. Oppress cis post] if they didnt put all." Meaning im ok with shaming these weird people who thing it's okay to be transphobic until they come to their realization that they need to stop trying to dictate people's lives (maybe not oppressing them tho).

People CONSTANTLY have stereotypes about races, ethnicities, ideas, groups, and mentalities. When they hate a group, they look to a minority of weirdos to justify their stereotype and opinion. But when it comes to groups they belong to, they want to destroy and shame anyone who makes light or has a stereotype of them.

When someone does wrong and belongs to a group, I'm ALL for shaming them. But please dont paint them as the figure head of a group to justify your hate. Even in this situation, plenty of cis people support people who are trans. Cisgender people are those who correspond with their biological sex— there are PLENTY of these people in the world. You can look to the US government for that or for really any humans rights groups. Maybe even you or your friends are cis. To say all cis people are shit is like saying all trans peolle are shit. It makes you no better than the group you demonize because you engage in the SAME BEHAVIORRR!!!

So allow me to ask— what's wrong with saying "not all ____ do ____?"
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TL;DR
People said I need to get outta here with the "not all cis bullshit" i spouted. They accused me of being "not all men" supporter, and they were right, but I also support not all women, bisexual, homosexual, etc [insert group] is doing [harmful stereotype]. More context above. Stereotypes in general are usually harmful and really not useful at all.

I want to know why saying not all _____ do ____ is so bad?