Lots of reasons but it's generally by women who lack a partner or don't have thier emotional needs met. They don't feel threatened by the child and confuse thier mentor/pupil relationship as love and as such doesn't see thier abuse for what it is.
Source? I’m sorry, but that kind of sounds like you’re letting female sexual abusers off the hook a bit by arguing their actions are innocent in their own minds. I don’t think these women are that stupid or deluded. I agree they find boys non-threatening, but I don’t think they construe their own manipulation of a young, naïve teenager as completely innocent. They know they’re taking advantage, they just don’t care.
But for female teachers, other motives may come into play: the desire to “care for” a male child from a broken home, which then de-evolves into sex; a need to feel attractive, wanted, and sexually desired by a young man, who may not have the capacity to understand or give real love, but certainly has the testosterone to provide sex; or even as revenge against a husband who is no longer emotionally supportive.
"Heterosexual nurturers” are women who develop sexual relationships with children and adolescents on the basis of feelings of love or a desire for intimacy. These women also often mistakenly assert that their behavior is not abusive.