Given your stated constraints, your best option is to re-phrase whatever it is you're writing to be gender-neutral unless you know what gender they want to be called in each instance.
If writing in English, remember that singular “they” is grammatically fine as a gender-neutral third-person singular pronoun.
A good example is the title of this question. As is currently:
… mapping a person's name to his or her sex?
That would be less awkward if written:
… mapping a person's name to their sex?