Hasn't Kaguya, after drinking the Hourai Elixir and living on Earth for so long, already lost her purity?
...That sounded wrong.
Well, she took the elixir on the moon, so her body would have been frozen (as it were) in a 'pure' state, I suppose. As an alternative, it's entirely possible that she's trying to pull something on Mokou.
...it is impossible for me to love.As a Lunarian, being without impurity...