Not everything operates at the individual level. For instance, dogs which have one gene for undescended testicles are more fertile than dogs that are aren’t. People who have one gene for Sickle Cell Anemia are resistant to malaria. Natural characteristics can have more than one effect and one can be bad for the individual but good for the population.
There is no evidence that homosexual behavior has anything but deleterious effects. Yet if some believe if it occurs because of natural impulses, it must be good.
Those are physical traits which, as you said, had in some sense a natural positive impact on sustaining life. Homosexuality presents no such benefit.