Sexual conflict can lead to members of each sex evolving traits to make their preferred outcome more likely. For example, females where producing many offspring is not desired, such as humans, have evolved to have elaborate reproductive tracts which pose an obstacle for sperm. These elaborate tracts also ensure that only the fittest sperm, with the most desirable traits, fertilizes the egg, leading to the best possible offspring.
While sexual conflict is common, it does not exist in all species. Species that exist without sexual conflict usually have docile females willing to submit to male dominance, such as rabbits and wolves. There are some exceptions though, such as spiders and ants, which live in female-dominated societies.