I actually have been considering asking this myself. The reason I haven't is its a complex balance to strike.
Case and point, the Hornet Hive Dungeon.
LOGICALLY there is no way Azyrran should trust someone who is a Dark Knight, who radiates corruption, and regularly sleeps with the Alraune and does other Corrupted things. And even if she couldn't tell you are horribly corrupted (which she probably can) their is little reason for a Dark Knight Character to purify the Hive, rather then just take it over and have more corruption.
But writing a dark ending to the Hornet Hive Dungeon, and having a second evil way to find it, requires more time, more coding, and more frustrations, for very little gameplay difference. Especially since even people who are playing a pure playthrough still want to have sex with the corrupt Hive Queen.
So in most cases its not worth it.
The second instance of why you need to be careful is be exhibited in Tel'Adre and the Demon Factory from CoC1. Now IN THEORY unless you cheat, it is highly likely you will find both Tel'Adre and Marae before you get too corrupt to unlock the first one or progress on the second one. However if you don't, either through cheating or accident, in the case of Tel'Adre you are now locked out of a huge amount of content....and in the case of Marae, you now have to grind your Corruption down via Jojo.
How this can be applied is the Kitsune Den. Its somewhat questionable that a Dark Knight player would be allowed to just wander around the Kitsune Den after defeating it, even if it turns out they had a valid reason, as Kitsune can probably sense corruption. However blocking players from the Kitsune Den for high corruption would lock players out of a bunch of content (even if not as much as Tel'Adre ended up having).
The same case can be made with Cait and Kiyoko. They should be able to sense corruption, and leave the party if the player gets too corrupt, but right now we aren't exactly TRYING to make the player loose companions. There aren't exactly that many replacements yet.
Still I think they were a little conservative in their approach with the Kitsune Den, as its hard to justify someone who has fucked their way through every other dungeon thus far, to not have the option to fuck the defeated Kitsune. This was likely done to justify why they were allowed to wander around after the fact....but it feels REALLY ODD that you can't fuck the Kitsune until after the dungeon is over given you are able to do similar things to in the Hornet Hive Dungeon, the Orc Dungeon, and the Corrupted Glade Dungeon.
Admittedly you can't fuck most of the Ghosts either, but this feels slightly more reasonable, because Ghosts.
HOWEVER it is worth noting that while in CoC1 you could fuck your way through every dungeon, their were some things that you couldn't corrupt, no matter how evil. (The best example being Scylla)
Regardless, I can understand why people seem to think that being a DK has no influence, because in CoC1 being heavily corrupted unlocked scenes and companions not available to the pure players, while locking you out of others, while being pure could unlock scenes and companions that the corrupt would never see.
Apart from what I stated above their is 2 other reasons why this has yet to appear in CoC2. Most of that stuff (excluding one weapon for the pure, and corrupting the farm for corrupt) was found in either the advanced zones, (the Deep Wood, the High Mountain, etc) or the Dungeons (And excluding the Secretary Succubus, not instantly attacking you it was only the late dungeons) These zones have yet to appear in game yet, so the content likewise won't be there.
It was also added (with the only exception being the Pure Sword) EXCEPTIONALLY LATE in the development cycle of the game. Even the corrupting the farm, which in theory could be done at a low level, was not added till most of the game was done.
Now some people at this point are probably going to add, "Hang on....CoC1 only had 4-5 dungeons. And this game probably has almost the same amount of stuff"
A fair point, but I think at this point its clear that CoC2 is going to a) Be a bigger game... and b) Be more dungeon/quest based then CoC1 was.
FINALLY, their is the nature of the hero. In CoC1 you were some guy. You weren't trained, you had no idea what was going on, and as we found out latter, you were (probably. unreliable narrator does apply) a sacrifice. So falling to evil is easy to explain.
The hero in CoC2 is still young, but has a year under their belt. They are experienced, and know the ways of adventure and combat. They chose this path, rather then being pushed onto it, and as such falling to evil and corruption takes more time....REGARDLESS of the fact that if you try really hard you can push that corruption meter up really fast.
Still think I should have been given the option to fuck the Kitsune mid dungeon.