I’ve never really experienced this at all, where people think you like all anime and give you stuff.
What I always experience are people sayings like “I don’t like anime” (also “comics are dumb”), which basically implies the same mindset. Because the same people wouldn’t say “I don’t like movies.” That would be weird. Like… which kind of movies? ALL movies? Really?
It is okay to not like a medium (like some people genuinely don’t like the medium of comics for various personal reasons), but I’m talking about when people clearly think all anime is the same genre (generally shounen). It’s a little odd.
At the same time, I think it can be shorthand (intentional or not) for not liking really common aspects of something. Like, when I say I don’t like bollywood music, what I really mean is that the artificially high female vocals drive me crazy. In a similar way, Japanese women intentionally sounding like babies (no matter what age they’re playing) really drives some people away. Then there’s other things, like how Japanese media generally doesn’t have the consistent tone that Western audiences enjoy. They don’t really have a tone problem, but it can feel like they so when you’re uninitiated.
Also every damn show is in high school
So I don’t think it’s totally invalid either. So I usually just point out that everything has exceptions and everyone is impressed by Akira 