Why is one occurrence of “born that way,” seen as just that, while another is seen as a mental sickness?

Two cases of “born that way”, but one is seen as a mental illness and the other not. From a scientific standpoint, how does one justify or reconcile that? If one is born with the attraction for the same sex, but another is attracted to the opposite sex, but a very younger version of such, they are both de facto “born that way”. Why is one a sickness, or a mental illness, hard evidence, not psychobabble opinion, and not a suppose illness anyone cares to treat less by death of the afflicted?