Yes, a perennial. One obvious solution (and my science students like cut and dried solutions), is to use plurals - not of course always the meaning one wishes to convey. But there is a move among grammarians/linguists in Australia to simply accept that their/they are third person singular non-gender specific pronouns. I'm sure that it will come. My aging Macquarie Dictionary (the Oz OED) says that it is 'usually considered to be bad usage', BUT I think that I've read that the Macquarie editors are thinking of accepting what is (arguably) inevitable. Does any-one have any info on this?