Homosexuality is not genetic nor a choice. It is the result of both genetic and environmental factors. According to Southern Poverty Law Center:
Modern science cannot state conclusively what causes sexual orientation, but a great many studies suggest that it is the result of both biological and environmental forces, not a personal "choice." A 2008 Swedish study of twins (the world's largest twin study) published in The Archives of Sexual Behavior concluded that "[h]omosexual behaviour is largely shaped by genetics and random environmental factors."
Dr. Qazi Rahman, study co-author and a leading scientist on human sexual orientation, said: "This study puts cold water on any concerns that we are looking for a single 'gay gene' or a single environmental variable which could be used to 'select out' homosexuality — the factors which influence sexual orientation are complex.
And we are not simply talking about homosexuality here — heterosexual behaviour is also influenced by a mixture of genetic and environmental factors." In other words, sexual orientation in general — whether homosexual, bisexual or heterosexual — is a mixture of genetic and environmental factors.