It is genetic in a number people but environmental factors play a big part. More than one thing can cause it. One reason: Later children in large families are

It is genetic in a number people but environmental factors play a big part. More than one thing can cause it. One reason: Later children in large families are more likely to be gay but the reason might be related to the mother already being exposed to hormones with earlier children. On an evolutionary scale this might make sense that the first born are more likely not to be gay as to pass on the genes but later children population control might make more sense. Another might be hormone levels of the mother. Another might be gene related where physical changes are different Another might be gene related that increase gender fluidity which combined with society can shape sexuality But we do know there isn't a single simple gene.