This code system https://fhir.yhcr.nhs.uk/CodeSystem/Interweave-SexualOrientation-1 defines the following codes:

CodeDisplay
1 Heterosexual or Straight
2 Gay or Lesbian
3 Bisexual
4 Other sexual orientation not listed
U Person asked and does not know or is not sure
Z Not Stated (Person asked but declined to provide a response)