Someone who is "legally married" under civil law may opt to live like brother and sister because they consider their interior spiritual life to be superior to their carnal life; they consider the teachings of Jesus to be above the precepts of the civil law; they identify primarily with their spirit, and wish to live the discipline of spirit over flesh.  The same reason that married people choose to be monogamous, even though it might be fun to sleep around; the same reason men and women accept celibacy when they take up vows, etc.