The reason you can use “they” in some circumstances (e.g. where the referent is indeterminate) is simply because of its common idiomatic use in this way. But there will still be many cases where you have to restructure the sentence in another way: and the reason for this is that, strictly speaking, English simply doesn’t have a gender-neutral singular pronoun.

I don’t mean to be pedantic, and I’m totally on board with using the singular “they”. I’m just pointing out that it’s not nearly as simple as saying that this is “grammatically unproblematic” as a general solution, or that “we have a perfectly good gender-neutral singular pronoun”, etc.

To parachute in a phrase from Hauerwas: where the gender-neutral singular is concerned, writers have only tactics, not strategies.

“The farmer’s breakfast has been eaten.” or even easier, “The farmer has eaten breakfast.”

The examples you gave where “they” would be used as a “singular” are all with reference to particular people…not to a general humanity! Of course it would be ridiculous to use “they” in any of those instances because your spouse is not a “they” but a particular person; the only reason to use they in these instances, I would assume, would be to do so intentionally to shake up a given identity with a presupposed gender…but that kind of thing doesn’t seem to be what folks are talking about here (I’m sure folks would shake their head in shame as they whisper to themselves: “identity politics”).

But, speaking of that sort of thing, the use of “s/he” is used in some feminist theory to refer to a performative blurring of identity, for instance in a transgendered person, or someone in drag — a specific way of saying that the typical assumptions of she and he here are performatively confused (cf. Judith Butler and at least one article by Mary McClintock Fulkerson where she deploys Butler’s understanding of performativity in a discussion of transgendered/drag feminist reflection). In other words, this is not as anathema to me as it is to you, Ben.

Personally, I’d be glad to see more use of the singular “they”. Certainly no editors or publishers anymore should accept the horrid 1990s usage of he/she, or (worse) s/he. (Any writer who does this should have their book contract torn up: note the singular “they”.)

And of course only a person of depraved morals would want to revert to the masculine generic “he” (which is grammatically unproblematic, but ridiculous nonetheless). But really, I think one of the biggest problems these days is the unthinking conversion of all singular statements into plural: the NRSV is full of indefensibly bad translations of this sort, e.g. where Jesus’ specific address to an individual is turned into some insipid general principle.

Still, the singular “they” is still very complicated, and there are numerous circumstances where it can’t be used correctly. Some examples: “My child brushed their hair”; “The student put on their shoes”; “The farmer ate their breakfast”; “The nurse decided they would get a haircut”; “My spouse looks hot in their new jeans”. (Actually, I just found this excellent wiki page, which has some very detailed and insightful analysis, and a summary of the many complications.)

There are technicalities about using the singular “they” where the referent is indeterminate, etc. But I think the main rule is just to use it idiomatically, i.e., where it sounds like normal spoken English. It definitely can’t be adopted as a simple generic solution for all sentences. If it sounds wrong, it probably is; every writer should try to make “their” writing more idiomatic.

Okay, that’s my two-cents worth. (Sorry, I taught grammar at university for a few years, so I find this stuff perversely exciting and fascinating.)