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.

If someone is attracted to both males and females - they are bisexual, regardless of what they say. If someone is attracted only to same sex as they are - they are homosexual, regardless of what they say. If someone is attracted only to opposite sex - they are heterosexual, regardless of what they say.

It is same as numbers. It is just describing what there is, it is not identity or something social, it just describes something that exist or happening. If animal likes to have sex with both males and females - that animal will be called bisexual, why humans are receiving different treatment? Humans are very special only in religions, but in reality we are not that different from other animals.