Social living can provide multiple benefits, but it also has costs. For example, male sexual harassment affects negatively female performance. Females can resist against males, but it frequently causes substantial costs to both sexes. How gregarious animals minimize sexual conflicts without losing social benefits is poorly understood. We show how a gregarious locust species, Schistocerca gregaria, reduces such costs. Our field surveys in the Sahara Desert found that sexually mature locusts form either female- or male-biased groups, i.e., females may prevent male sexual harassment by localizing separately from males. Egg-developing females lived in female-biased areas. They visited male-biased areas when they were gravid and were mate-guarded by males before oviposition. Male mate-guarding prevents attacks by rival males, thus, assuring safe oviposition. Males aggregated on the ground and mated with incoming gravid females. The last copulating male’s sperm is used for fertilization, so males must guard their partner until oviposition to assure paternity. Although intra-sexual competition was high in male-biased groups, males encounter receptive gravid females, resulting in reduced mate-guarding times and increased a chance of mating. Therefore, this mating system may allow both sexes to reduce the costs of sexual conflicts even in large groups.