To add to that…
"Gay" is not a scientific term. You can distinguish between ABO bloodtypes with a test. You can consistently measure Ph with a test. But sexual orientation?
One guy might describe himself as 'Gay' without ever having had sex with a man. Another guy could have frequent sex with men and say he's Straight. A third guy could say his culture doesn't use that word. Another guy could say he's a pre-op Transwoman. A woman could say she's a pre-op Gay Man.
Even if you clear gender out of the way; Gay men aren't all equally attracted to the same type of men. Someone could be attracted only to Muscle Bears and have zero attraction to Twinks. Is he the same 'sexual orientation' as someone who is only attracted to Twinks? I've had friends where we were NEVER attracted to the same guy.
A piece of litmus paper doesn't react to certain types of acid but not others on a near random basis.