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.