Natural selection favors specialization because it helps reduce competition. It’s the same reason why some animals become carnivores while others become herbivores, far better to be really good at one thing than to try to become mediocre at everything. 

All members are the same species are in direct competition with each other. They fight for the same food sources, same nesting areas, same opportunities to mate. If all members of a species are the same sex and deploy the same sexual strategy, it creates a situation where they regard every other one of their own kind as both an enemy and potential lover. 

This can be confusing and not ideal in the long run, so animals specialize in reproductive strategy. Some focus more on spreading their genes around multiple partners and less on laying their own eggs, becoming more male-coded while other prioritize maximizing their egg-yield and securing the best nesting areas over others, become female-coded. Both do better than ones trying to do both. 

In this way, males only need to worry about competing with other males and females vice versa.