I suggested to my dad we watch Avatar: The Last Airbender.
He asked whether I was talking about the cartoon or the movie, and I answered that the cartoon, of course.
"Are you crazy? You're not a child anymore!"
...?
I tried to explain that just because it's a cartoon doesn't mean it can onky be watched by children, but he seemed to ignore me.
Aww dude, that sucks. I've been there too; have a hard time explaining to some folks that animation isn't all My Little Pony, despite the obvious counterexamples. I mean, stuff like Rick and Morty or Southpark or Bojack Horseman have been around forever. Even oldies like Rocky and Bulwinkle can be appreciated for it's silliness and wit, right? It's hard to explain what's worth watching in something like Avatar: The Last Airbender or Steven Universe to folks who automatically dismiss them as kids stuff. Sometimes I say that it's like Pixar animations, in that there's layers that people of any age can appreciate. Even the cuteness, or the action, or the absurdity, or the aesthetics and writing technique, or whatever else is plenty enough to enjoy it. If you're willing to watch vacuous Horror or Action Schlock, you can watch Cartoons dangit!
There's a flip-side too; my Brother-In-Law won't watch early Miyazaki with me either, though I know he'd like it. He just says "Isn't anime the Tentacle Porn thing?" if I bring it up. It's probably more a joke than anything, but it still sucks to be dismissed, you know? :<
Whatever it is, it's crappy to want to share something you liked with someone else, only to be undermined by whatever preconception they had. Best thing for it is probably to seek out some folks who will share it with you, and who won't undermine you or make you feel crappy for liking the things you like.