Ch. 2b. The reason becomes clearer.
In the Magic Box, Willow was now very close to tears as she tried to explain to Giles how she got so messed up "I, I was envious of Buffy, Giles… I mean, her abilities for the cause of righteousness are, quite frankly, astonishing. But, I know, ladies- such as I'm called to be, don't attempt violence from envy… at least proper ones don't! Then, the solution seemed to fall in my hands: magic- I studied all kinds of spells- some seemed too brutal: blood sacrifice, and so on… My affection for Buffy is that of a twin- but I digress, then I found a type of magic that didn't involve murder or blood sacrifice of an animal, so I was overjoyed." "What kind was it?" Giles asked, really concerned. Willow's voice dropped to a whisper and she hung her head "sex magic" she said, ashamed "using my body's reproductive system to gain spiritual power- it seemed to be the gentlest means of doing what I wanted to do." Rabbi Micah was aghast, now it all clicked "Satanic pornography" he said, utterly disgusted "You've delved into THAT GARBAGE?!" "I tried to warn her that she might get addicted- just never thought that she'd go that far." Giles said. Now tears were flowing down Willow's face openly, but it was Xander who walked up to and held her as she cried her eyes out from shame.
Micah motioned Giles away to the front door and with a flat wave of his left hand he said "Hardly anything you can do now- but you've made quite clear that you have a father's love for her, and that's exactly what she'll need if she's to pull through this." and to Willow's parents "I don't know as your daughter will ever be the same again- she's unbelievably lucky that she didn't wind up possessed by the Devil or one of his lieutenants!" and to Willow "If I wasn't convinced that you needed religious help before, I am now… the offer still stands, interested?" "Anything, sir." was Willow's response. Resembles Cain and Abel thought Micah.