The word ‘they’ has two legitimate uses and one illegitimate use. The first proper and legitimate use is as the subjective case of the third person plural pronoun – that is, to refer to a bunch of people. The second is as a singular pronoun in cases where it is impossible to know the gender of the person referred to, for example: ‘If someone wins the lottery they should…’ In such cases, and only in such cases, it is proper to use ‘they’ just as ‘you’ has been employed for centuries – covering both singular and plural.