Just so it's been put into words at least once in here, when people start to talk down to you you can just move on and ignore them. Plenty of people in this forum don't actually want to engage and instead just pull a random opinion card from their deck of default responses, post it, and then when pressed pull the next card.
You're not going to get actual answers from people that just go and tell you you're brainwashed, because the point isn't to have a dialog, the point is to exhaust and overwhelm you.
It's a very effective technique and one you'll have to get familiar with, it's also often used as part of propaganda campaigns for various purposes.
As such, if you have to spend an undue amount of time asking and get nothing of substance back, discard what they say and move on. If it still intrigues you you could try having a search elsewhere about some of the points they've raised, or better yet ask one of the adults in your life that you trust to give you the down low on whatever the hell they've said. Particularly if they're internet savy they can probably explain wtf is going on.
For this particular topic, the "brainwashing" is part of an ongoing culture war the alt-right in the US is having with everyone else, where sex education and any talk about gender identity is being labelled as grooming to force people to think that everything relating to LGBTQIA+ topics is inherently of sexual and predatory nature. This is simply a ploy to stigmatize all related movements and topics and negatively taint all discussions and topics of it, making it an easier enemy to gain votes on for various elections.
You likely won't have this as a topic in school but similar shit has happened over here in Germany where the nazi party hunted down queer people for existing, burned all research on sexuality at our largest university and research institution for sexuality
(link), and then ultimately put a lot of queer people right into concentration camps together with the other undesirables like Jews, Roma, Sinti, autists, and basically anyone else that wasn't aryan.
If you want to have a more in-depth primer I would tentatively suggest giving a watch to Some More News recent video about that same topic: "Who Are The Real "Groomers"?". which is effectively a vlog working through that entire topic. However it's aimed at millenials, the jokes are crass, the host is bearded, and it's rather dense, depressing, and long due to the topic.
And now I wait for the usual suspects to throw the usual tantrums about the "bad" word I used above.