Persons may or may not express their sexual orientation in their behaviors. In most people, sexual orientation is shaped at an early age. There is also considerable recent evidence to suggest that biology, including genetic or inborn hormonal factors, play a significant role in a person's sexuality. In summary, it is important to recognize that there are probably many reasons for a person's sexual orientation and the reasons may be different for different people. No, human beings can not choose to be either gay or straight.
Sexual orientation emerges for most people in early adolescence without any prior sexual experience.
Although we can choose whether to act on our feelings, psychologists do not consider sexual orientation to be a conscious choice that can be voluntarily changed. The reality is that homosexuality is not an illness. It does not require treatment and is not changeable. However, not all gay, lesbian, and bisexual people who seek assistance from a mental health professional want to change their sexual orientation. Gay, lesbian, and bisexual people may seek psychological help with the coming out process or for strategies to deal with prejudice, but most go into therapy for the same reasons and life issues that bring straight people to mental health professionals.