You seem to be a bit too ready to apply guilt by association in regards to MLP. Every community for everything ever has its outlying strangeness, trolls and fetishists. Really, you can Google R34 anything you will get results. If you go to more moderated sites like Equestria Daily, then you won't see any of that sort of thing.
As for why normal teenagers don't know about such things, I would imagine it's because they base their interests upon what other people like, as the purpose of having such interests is, to them, to be able to interact with other people whilst doing those interests and to discuss them. Personally, I am usually interested in things on a purely personal level and I generally prefer more solitary activities. Reading, writing, playing video games, that sort of thing. Video games have become more of a social hobby in recent years, but I've been playing since the Sega Mega Drive/Genesis and have gone further back than that with GOG and emulators.
About the things you mentioned interest in, I found Monty Python hilarious, have heard about the Colbert Report and its excellent parody, generally prefer instrumental music like rock, metal, even classical, and don't really recognise most of the movies you mentioned.