Freud (1905) believed that every human was innately bisexual, and that monosexuality (i.e. hetero- or homosexuality) is a result of internal and external influences during psychological maturation.
Currently, the question of a biological, e.g. genetic or uterine, cause of non-heterosexual sexual orientation is still a subject of research and has not yet led to any conclusive results. Until that question has been answered with some level of certainy, we simply do not know whether homosexuality is a natural variety of sexuality or a deviation from an inborn disposition through influences after birth. (Wikipedia summarizes the research well: http://en.wikipedia.org/wiki/Biology_and_sexual_orientation)
What you must differentiate is homosexual behavior, which can appear in heterosexual individuals such as experimenting adolescents or prisoners, and homosexuality, which is a preference for same-sex partners, which might not show up in behavior at all. Historically, many homosexuals led heterosexual lives, marrying, mating with and procreating with opposite-sex partners, despite a contrary sexual preference.
- Freud, S. (1905). Drei Abhandlungen zur Sexualtheorie. Leipzig: Deuticke.