She's leaving. She doesn't want to, but she doesn't really have any other options. Willow's gone too far this time. What was she thinking? How could she not see that Tara would be hurt by something as personal as that? God, what she did... it was worse then what Glory did to her. Because she loved Willow. Trusted her. And then she violated that trust, in a way so intimate that she can't even think about it without crying.
It's like being back home with Donny and her father, only Beth isn't there to help her through it.
And she knows what she's doing is right. If she stayed, she'd be condoning everything Willow did.
So she'll walk past Dawn, biting her lip until it bleeds, and not go back. Ever, if necessary.