Shizuru, I think, felt that just loving Natsuki the way she did was molestation. She was betraying Natsuki's trust in her friendship by lusting after her, dreaming of her, fantasizing about her.
Shizuru is, in some ways, a girl wise beyond her years. She's intelligent, capable, powerful. But she's also just a 17 year old girl in love with her best friend (one of only two real friends we know she has), a friend with a lot of emotional issues at that.