Nobody knows what causes homosexuality.
The following is a quote from the American Psychological Association on the topic:
“There is no consensus among scientists about the exact reasons that an individual develops a heterosexual, bisexual, gay, or lesbian orientation. Although much research has examined the possible genetic, hormonal, developmental, social, and cultural influences on sexual orientation, no findings have emerged that permit scientists to conclude that sexual orientation is determined by any particular factor or factors. Many think that nature and nurture both play complex roles; most people experience little or no sense of choice about their sexual orientation.”
“Research over several decades has demonstrated that sexual orientation ranges along a continuum.”
There is a myth out there that science has demonstrated homosexuality to be “genetic” or biologically “hardwired” and that orientation is like a switch either on or off. Neither are true.