The problem starts with the Manichean fallacy (the either-or fallacy)--because of the sexual reproduction system of mammals, we take for granted that because there are two parts to reproduction, there are also only two sexual attractions. But physiologists have shown that the hormones (testosterone, etc.) that work on the mind and impulses are found in us all--in varying proportions and percentages. What has been proven is that sexual preferences are not chosen by a conscious decision; lifestyles,however, are chosen.
There is no know trait in humans that has been associated with homosexuality. There is no main cause because its not an ailment or disease. Sexual awarness does not come into effect until adolescence even though there are some who are sexually aware before adolescence. There have been studies in other animals where changing phermone receptor has changed animals sex preferences.
There has been a genetic study in fruitflies where scientists changed the receptors for female phermones in male fruitflies. Results showed that males were attracted to male phermones.However, these types of studies have not been conducted in humans.