Why do people think my sexuality is a mental disorder and/or invalid?

In my experience, there are people who simply cannot comprehend the idea of feeling like you are anything other than cishet. They are so strongly in the cishet bubble that to consider yourself to be anything else "can't" be natural in their minds. You may as well tell them you have the power to turn invisible, but only if nobody's looking. Some people can get to at least a basic level of understanding and acceptance, but some people will never be able to get past their pre-established worldview.

Maybe because they can't accept the truth or see what's right in front of them. Or maybe because they can't accept it for they've been hateful and close minded for so long.

It's hard to know exactly how to answer that without knowing more details, but sometimes people have trouble understanding things they have never felt or experienced themselves. People often have feelings based on what their heard growing up, and aren't always open to the ways in which sexuality is becoming more expansive/fluid. It's probably more a case of ignorance than any personal feeling against you!

Because of either ignorance and prejudice. The only way to fight it is by spreading awareness about the different sexualities, both in scientific and human terms, in order to spread a mentality that is aware that all orientations are normal and that prejudice hurts people. Some won't be willing to listen, but some people will, since not everyone has been given an opportunity to learn more before, because of their upbringing. It's an effort worth making, a better society depends on all of us.