Sexual conflict is thought to be an important evolutionary force in driving phenotypic diversification, population divergence and speciation. However, empirical evidence is inconsistent with the generality of sexual conflict as enhancing population divergence. Here we demonstrate an alternative evolutionary outcome in which sexual conflict plays a conservative role in maintaining male and female polymorphisms locally, rather than promoting population divergence. In diving beetles, female polymorphisms have evolved in response to male mating harassment and sexual conflict. We present the first empirical evidence that this female polymorphism is associated with 1) two distinct and sympatric male morphological mating clusters (morphs) and 2) assortative mating between male and female morphs. Changes in mating traits in one sex led to a predictable change in the other sex which leads to predictable within-population evolutionary dynamics in male and female morph frequencies. Our results reveal that sexual conflict can lead to assortative mating between male offence and female defense traits, if a stable male and mating polymorphisms is maintained. Stable male and female mating polymorphism are an alternative outcome to accelerating coevolutionary arms race driven by sexual conflict. Such stable polymorphisms challenge the common view of sexual conflict as an engine of rapid speciation via exaggerated co-evolution between sexes.