The problem : Third person singular pronoun for unknown gender is missing .
Here is a situation to explain the case .
You are waiting for the delivery of your package in mail. You and your friends are at your place and talking about when it may arrive. You tell your friend I don’t know when he will be here. You just made some women upset by sounding one-sided on gender issue.
The problem here is that you don’t know the gender of the person who will be delivering the package , thus saying either he or she would not only be logically wrong but also sexist . Because if you use only he then it means that you are neglecting a possibility that there could be a woman delivering a package and vice versa.It would also be incorrect to use they for they is plural and as you know it’s going to be only one person who will deliver the package.
The grammatically correct way to say this would be I don’t know when he or she will be here.While it is a politically correct way to speak a statement it also makes it difficult having to use that each time you make a statement and may sound funny and strange to some.
I hope I explained the situation well and may be you have already come across and thought about this issue specially if English is your first language.