Originally Posted by Wikipedia

Research has identified several biological factors which may be related to the development of sexual orientation, including genes, prenatal hormones, and brain structure. No single controlling cause has been identified, and research is continuing in this area.

The prevailing view is that sexual orientation is biological in nature, determined by a complex interplay of genetic factors and the early uterine environment. Some believe that sexual orientation is established at conception, therefore not a choice. That is, individuals do not choose to be homosexual, heterosexual, bisexual, or asexual. There is no substantive evidence to support the suggestion that early childhood experiences, parenting, sexual abuse, or other adverse life events influence sexual orientation.