Isn't it painful to see “they” used in the singular?

You should not feel any pain from the use of “they” as a singular pronoun—for instance to refer to a person of unknown or unspecified sex—since it is perfectly correct English.

Not only is this use very natural and common in spoken English, but in written English it is acknowledged by the OED. Singular “they” (or “their” or “them”) appears in Shakespeare, in Chaucer, in Spenser, in Swift, in Defoe, in Shelley, and in Byron. It was used by William Thackeray, by Walter Scott, by George Eliot, by Jane Austen, by Charles Dickens, and by Robert Louis Stevenson, as well as by George Bernard Shaw, Lewis Carroll, Oscar Wilde, Rudyard Kipling, H.G. Wells, W.H. Auden, George Orwell, and C.S. Lewis. American writers who used “they” in the singular include Walt Whitman, Edith Wharton, and F. Scott Fitzgerald.

On the face of it, if you were inventing a language, you would certainly want to create a pronoun which referred to a person (thus distinct from the dehumanizing pronoun “it”) but which did not imply a particular sex, since the sex might be unknown or indeterminate. So it is hardly surprising that English has actually had such a pronoun for centuries.

It is true that, without the aid of context, “they” does not distinguish between singular and plural referents. But “you” no longer distinguishes between singular and plural either (now that we've given up “thou” and “ye”) and no one gets themself into a tizzy about that.