Primogeniture makes sense in a world where odds that you arrive at the adult age are rather low, due to the high risk of death from illness and injuries.
The oldest child is the most likely to survive. It is a rational and fair rule in such context.
Is the oldest child the most likely to survive, or is the winner simply the oldest surviving child?
Many discriminatory policies could be considered rational. It is rational for jobs to discriminate against handicapped people who require extra affordances to be able to do the same job (for example, wheelchair ramps).
“Fair” is a much trickier beast! My favorite approach to conceptualizing fairness is Rawls’ veil of ignorance: if you were going to be placed as a random member of society, rather than your current position, and you would still support a policy given this change, then the policy is fair. Knowing that, beyond the veil of ignorance, I may be a paraplegic, would I still support dismantling the ADA’s wheelchair accessibility requirements?