Well, I don't tell them "no, you're bisexual," because it's not worth the effort. But I roll my eyes and mentally put them in a box of "people I'm never going to try to have a real conversation with." The last few years have made me understand deep in my bones that people who want to live in their own little reality away from everything else are actually dangerous. Half a million people died last year in the US alone who didn't have to die, because of a pervasive cultural attitude that it's just fine for individuals to have their own alternative facts and alternative reality.