"Why must every single human being be sexual?"
I didn't say that we must. I did say there is no scientific evidence for asexuality.
Why is impossible to accept that some are not, and are born that way?
It's possible, but not proven.
And why is it a problem to be fixed?
It isn't for you, but it IS for some people.
Is there “scientific” evidence of homosexuality?
YES!
I don’t need you (or scientists) to believe in me. I exist.
Yes, you exist as a person, but I am not obliged to believe in asexuality. I believe in your right to identify that way if you find it helpful.