Questions regarding biological sex seen as offensive?

I have been tasked with creating a new front-end for an insurance company, the idea being to provide rough quotes based on filling out a questionnaire, the back-end quote generation etc. is all already working and has been used internally for years, we plan to provide a significantly wider range than the quote provided by this system to the end user.

The questions are quite straightforward: age, location, ethnicity, smoker non/smoker, years driving etc. However, one particular question has given me food for thought for a while now.

The back-end system requires the person's biological sex as an input factor, because testicular cancer and breast cancer among other things are taken into account for the quote. I am worried that only having two options for sex may be seen as trans-phobic or exclusionary, an 'other' box however is not adequate, as we do need to know their biological sex.

The current WIP is to have a 'Sex' selection and an (optional) 'Gender' selection separately. Although this may be confusing for some, the ethical issue is that we discard the 'Gender' selection, basically making it a padding to avoid offending trans people. Which in my opinion is offensive, and the UX equivalent of giving a child your keys to make them happy.

I'd like to have a solution to this issue that fulfills these points:

• The user is prompted to provide their biological (birth) sex
• The user is not made to feel excluded if they are trans or otherwise
• The questionnaire is presented as ethically as possible

I believe this issue is unrelated to other "duplicates" purely because an 'Other' box etc. is not applicable, and the question cannot be removed entirely.