Th basic goal of all life is to reproduce. From single cell organisms to a fly to a human, the are born to reproduce. Some forms of life literally reproduce then die because their purpose was served.

So, considering homosexuals won’t reproduce, it very well is a mental illness because it goes against the main drive of life, something we are wired to do whether we want to or not. Bisexuality is normal along with being Hetero, but homo is not