saying that “I’m asexual (but sometimes I’m sexual)” is a pretty blatant and obvious self contradiction.

Except that asexuality is a spectrum and most people who aren't constantly horny fall somewhere in an area one could describe as a "blatant and obvious self contradiction" if we write off such grey areas as paradoxical. There's asexual orientations that are so normal most people never even get the idea that there could be a label for how they feel, like demisexual. It's actually really productive to let go off the notion that asexuality = robotic Mr. Spock kind of person. I mean, it's also a whole lot less offensive to ace people, but it's also kind of a prerequisite to understand the full extend of human sexuality and i don't mean that in a "understand a low single digit percentile of people who never get aroused and never have relationships" kind of way, i mean that in an "understand how the majority of people feels and frequently doesn't feel sexual desire" kinda way.