> What are the fairies up to at the moment? Consider both those that are all together and those whose spirits are separated from their bodies.
> Shrug.
> "We could also just tell Reimu about this and ask permission."

> Can we still hear the music?
> Are there any notable figures in the auditorium. Try to look for Youmu or Yuyuko in the area.

I think the reason the Lunarians were so averse to the impurity the fairies in LoLK produced was a matter of degree, not difference. Junko's power brought out the fairies' latent life energy/impurity to its greatest extent, to the point where the Moon gained a breathable atmosphere. Because the Lunarians are hypocrites, they're okay with going to the earth for a night or dealing with the occasional human or youkai, but this level of impurity was presumably a different story, to say nothing of what Junko herself could do if they fought her directly. Or it could be the amount of exposure would be beyond what they could tolerate if they had to wage a proper war.

Also, I would argue that anything we know about celestials cannot be reliably applied to theories on Lunarians or purity, as we don't know if they're pure beings or even if heaven is classified as a pure land. ZUN remarked in his SoPM interview that Lunarians are "closer to hermits or celestials than humans," which taken another way means that all three are different types of beings with their own distinctions.

We know remarkably little about heaven or its residents compared to hell or the moon, but it is known that celestialhood is something you're "promoted" into, and that enlightenment itself does not a celestial make. Long-time hermits like Seiga can be passed over for celestialhood, and untrained humans like Tenshi can be made into celestials if they're chosen for whatever reason, and the physical difference is large enough that they go from delicacies to poison for youkai. The process that makes one a celestial is completely unknown, but even if heaven were a pure land, there's no reason to think it is the same method the Lunarians took. My personal theory is that hermits and celestials pursue a different path towards agelessness than the Lunarians do, as neither care about impurity or spending time on Earth - just averting death itself. But there's such little information on them that I'd be cautious about making any assumptions one way or the other.

> "Not at all. I'm actually glad you got your own treasure out of this trip. Hopefully the trend continues."

I also think that purity (and impurity too) is complicated in Touhou cuz these words are not used in a very restricted manner. So I'm gonna try to help by focusing only on the Lunarians only here :

Whenever impurity has been mentioned, it's always been in relation to life and death.

There is actually also another suspicious brand(?) of impurity that the Lunarians are clearly not afraid of and is probably less related to life and death but probably more of it being impure as it's from earth : to illustrate, we have the Lunarian space probe from LoLK, which was virtually invisible to Youkai (who couldn't do anything about it) because the Youkai were too impure ; and we have the fact that Celestial flesh (actual pure beings) is toxic to Youkai and even have an uncomfortable presence to them ; and the fact that the Ultramarine orb's purification effect would instagib native gods such as Suwako ; and small speculation here but the possibility that the Youkai's defeat in the Lunar War, aside from technological inferiority, could have anything to do with their impurity imo.

1/All in all, the main takeaway here is the fact that basically impure beings are unarguably inferior to pure ones, and anything from the earth's make is impure (They even have shorter lifespans, inferior technology, probably dumber and stinkier too)

As for the moonies, since we've already seen the Watatsukis and EVEN Sagume personally go down to Earth despite it being rife with impurity (eww), I personally think the average unnamed Lunarian's distaste for impurity is more rooted in arrogance and ideology rather than in-universe lore understanding of it being bad for their health (not everyone should be on Sagume and Eirin's intellectual level anyway) : I think that aside from what WhitheringFlowerCrown said about Kaguya taking the hourai elixir, there shouldn't be a practical reason to ban Kaguya : it disconnects her from life and death anyway, so why ban her for impurity related pragmatic reasons ? It's gotta be ideological : she's basically failed the Lunarian ideological purity test by thinking like one of the stinky impure earthling, as in, taking the elixir cuz of mortality while she shouldn't even be afraid of that as a Lunarian.

2/So basically the average Lunarian's attitude towards impurity is that of a complete ideological rejection

As for the good old kegare, or the impurity of life and death that the Lunarians just can't handle, while I agree that it's probably still worth it to take Shinto Cosmology (rather than Buddhism) into consideration since Touhou lore uses traditional Japanese folklore, beliefs and religion as a backdrop anyway, I think it can be safely ignored since most of its relevant concepts are not really in use in Touhou : as far as this series goes, I think it's alright to simplify it as impurity from life and death.

But at any rate, when it comes to the difference between the moon and the earth in touhou, whenever we have something moon and purity related, it's generally about things staying the same : to illustrate, we have the moon's udonge tree for which blooming will serve to signal impurity so it's safe to assume they generally don't do that (cuz no impurity on the moon) ; and we also had Eientei be pretty much unchanging too before and during IN ; while in LoLK, the Entire capital was frozen in time to prevent impurity. Meanwhile, on Earth / Gensokyo we have Yukari's attitude in one of Marisa's LoLK ending where she's like "Gensokyo can change" (plus another FS chapter to that effect) ; and I also vaguely remember a Mamizou - Kokoro conversation where Mamizou tells Kokoro something that basically "life is about change" (although I can't find that conversation in the wiki ?? Srsly I remember something like that) ; and finally there's also Reisen, who changed from being a moon rabbit to a earth one during her stay in our stinky world : Thematically speaking, in Touhou the moon is always about eternity and the unchanging, while the earth is about change and growth.

3/Considering all that, I'd say that a pretty correct babyfied oversimplification is to say that impurity in touhou (the variety that Lunarians can't handle) is what allows change that lead you closer to death.