I voted for using the singular 'they.' As has been pointed out, language is a living thing and usage changes over time; 'they' fills a void in the language. And if we can have a gender-neutral 'he' why not a singular 'they'? Besides, basing your usage on whether the subject has dangly bits in their underwear ignores the fact that gender is not actually binary. And how exactly do you go about finding out? "Excuse me, I need to know what kind of genitalia you have. It's for an article I'm writing."