Then morality is anti-adaptive behaviour, morality will be selected against, and over time moral lines & populations will tend to die out and be replaced by less moral ones.
Of course morals are memes not genes, and like diseases, memes can jump from one population to another. Morality may be leading to population X dying off, but if population Y can be persuaded to accept the morality it may not replace X; and Y may eventually die off also.
Still, any morality that kills its hosts too quickly (eg Heaven's Gate cult) will indeed tend to die off.
I could ask you whether you think that, if morals are indeed not as conductive to the species survival, they should go away? But that's certainly a loaded question.
I tend to believe that moral behavior ultimately leads to good for the survival of the species. So any genetic traits that help us act moral or accept morals will survive for the future, and any that don't will be diminsihed, and the "meme" of morals itself will survive and change to make it better for us overall. But that's just a believe, I could be wrong.
Anyway, just because some our behavior can be explained by evolutionary concepts still doesn't guarantee it is good for us, and certainly isn't inherently a "moral" thing we need to keep around for that. Maybe gender stereotypes were a great thing for our survival when most of our daily activities involved physical heavy duty and was not so great for the survival of unborn, and there were lots of stuff to do to maintain a household (be it in a cave or a real house). It doesn't mean that it's great now, when we have dish-washers, microwaves, freezers, water pumps, textile factories, as well as pig farms, tractors, cars, planes or computers and generally a lot more free time than ever before.
So we have to evaluate the behaviors and traditions "evolved" in a different environment then our current ones, and adapt those that don't fit. In a way, we're doing natural selections work, based on capabilities we gained by natural selection in the first place.
Of course, that is not an argument against sexism within a setting. It's an argument against sexism today. An argument for sexism in fantasy is that
1) It has to find an audience today.
2) We can use a setting to explore the unknown - a different evolutionary path - just because we didn't take it doesn't mean it doesn't exist, particularly in a world of magic that can change around so many things...
3) When we do it, do it consciously, not just because your millenia old "reflexes" make you do it.
I suppose 2 and 3 are the same, in a way - use it because you want to use it for a reason.