For instance, specific scent glands emit odors that are exciting to the opposite sex, or internal secretions of other sorts may stimulate certain muscles. However, as we move up the scale, behavior that had appeared to be automatic and directional either disappears or begins to lose its orderliness. A centrally important evolutional change in all this is that the cerebral cortex, vital to the learning process, has slowly begun to increase in size and in its ratio to the rest of the brain. It comprises about 25 percent of the brain in lower animals, more than 50 percent of the brain in anthropoids and over 90 percent of the volume of the human brain. From this point of view it can be seen why man can learn as much as he does and, lacking the physiological control that are apparent in lower animals, must learn all of his behavior in the period of time his maturity requires.
People simply respond sexually in ways in which they have learned to respond.
Conversely, people fail to respond sexually when they have arrived at an aversion. Unfortunately the majority of human beings never live according to their own nature. Only a minority by-pass stereotyping by parents and society and live more authentically than the rest. Bisexuality could be that authentic lifestyle. However, the term bisexual is somewhat misleading.