Normally developed people don’t need to “decide” their sexual orientation. It grows in them naturally until it is fully mature. And it is unerringly heterosexual.
But if some trauma arrested a child’s sexual development before it had a chance to differentiate, the child may be (pathologically) homosexual. That is a mental illness and can be treated.
I know the exact moment a biological switch clicked over, and that I was aware of my hetrosexual orientation, and I was suddenly and vividly aware of the mysterious attraction of the opposite sex - it was not something that "grew" in me gradually, it was a strong biological propensity that expressed itself dramatically and without previous evidence.
Some gays and lesbians I know report a similar experience of instantaneous and vivid experience of attraction to members of the same sex, and I've no reason to suspect that their experience was any less "authentic" than my own.
Your claim is just a tautology, by your definition anyone who makes the discovery that their desire is heterosexual is "normally developed", anyone who discovers the opposite is abnormal because you define normal are "unerringly heterosexual". But that's just your definition, and I doubt it would be remotely convincing to anyone who's made the discovery that they are "unerringly homosexual".