Some people keep both same-sex sexual intercourse, also keep the opposite sex and sexual intercourse between is called bisexual people. Some people just in a certain period in life or in a particular environment to a tendency to gay, soldiers serving seafarers like sailing, or collective dormitory students, called situation bisexual; Others because of love, marriage, or for other reasons have occasionally same-sex sexual behavior, in order to seek the feelings of anger, illustrate the bisexual happened the day after tomorrow, it is influenced by personal and psychological factors.
Some people think, the occurrence of bisexual has a biological basis, is born with some nature. There are genetic factors, hormones and brain sexual differentiation factors. Others think that gay is the psychological factors and social factors, or the day after tomorrow, is raising process deviation. People who hold the idea after there is no enough evidence completely denies the existence of the biological factors. Some modern scholars for the above two tit-for-tat theory put forward another point of view, the biology and in the process of raising factors complement each other and influence each other and work together. This role will play a role in certain period, so called critical period. It stresses exist in the key period in the process of people psychological mature biological factors, genes (nature) and cultural environment factors (breeding, the day after tomorrow), and the function of interaction, and to the person's behavior or sexual orientation have a lasting memory mark or mark, remember the day of its critical period began in fertilization, continued until after puberty.
To undergo a series of critical period in the process of sexual development, or to undergo a series of mark or fork in the road. In other words, in the process of sexual differentiation and maturation has certain variability. When a person into the sexual differentiation of each mark or fork in the road, he or she has a two-way choice, can choose to be straight, may also choose to gay people or bisexual people. But when a man entirely through the gateway, the gateway will shut down, so its sexual differentiation or sexual maturity will be fixed in the previous development direction. However, a person in the first stage, there is a big sexual orientation degrees of freedom, like to run in any lane, but when a person is growing, will increasingly society according to their gender specified "lane", finally fixed on the "lane" defined by gender. This choice is permanent and will not change easily. That is to say, if became bisexual men or women, it is hard to change.