Of course, that's not the only way to do it. You can have more than 2 genders, and many species do. But specialized sex structures and body types come long after the genetic basis for sex in evolutionary history. It's interesting but it's not a major problem.
And you have species where sex is determined by external conditions and species where sex changes with maturity and species that change sex and species were a single organism is both sexes and ...
Aslan is not a
Tame Lion