Sexual orientation is not a binary outcome
Only things with binary outcomes can be reduced to genetics. Sexual orientation is not binary, it is fluid.
With binary outcomes, it is possible to say with certainty that a foetus, if brought to term, will be born with a gene that will affect its development or behaviour. With non-binary outcomes, it is not possible. Sexual orientation is not a binary outcome and is therefore not something that is dictated before birth.
Medical professionals can test the genetics of two embryos and tell expectant parents with 100% certainty that their babies will be identical twins. Being a twin is a binary outcome.
However, when it comes to testing for qualities without a binary answer, things become more difficult. For example, we cannot test the genetics of a child and immediately determine how tall that child will be. Height is a non-binary outcome and depends on the interaction of different genes, childhood nutrition, illness, and environmental impacts.
Same-sex attraction is not a binary outcome. Homosexuality is a sliding scale and is, like someone’s height, not something predetermined before we are born.
[P1] Only binary outcomes are predetermined before birth.
[P2] Sexual orientation is not a binary outcome.
[P3] Therefore, people are not born gay.