Ninety percent of the species were female and they supplemented their gene pool by mating with members of other species. They achieved this by releasing a retrovirus on nearby planets that infected the host male and began adjusting his DNA. The host then started to become Taresian and was drawn to Taresia, where he was welcomed and told (incorrectly) that he had always been Taresian and that the species reproduced by implanting Taresian embryos in females of other species. The unsuspecting males were then separated from their shipmates and their every whim was catered for by the women of Taresia. Each male was then married to three females, who mated with him. Unfortunately, the amount of DNA removed in the mating would kill the unsuspecting male, hence the Taresians' elaborate deception.
This deception was uncovered by the crew of the USS Voyager in 2373 when Ensign Harry Kim was infected by the Taresian retrovirus. He discovered the body of one of the previous males and escaped the planet.
Because the nearby Nasari disapproved of the Taresian mating process, the Taresians were at war with the Nasari. Despite the Nasari being quite powerful, Taresian ships were quite capable of holding them off. (VOY: "Favorite Son")