I think it might have more to do with the instability and lack of connection a non married partner has to his or her partner’s children than to sexual orientation. But I would be wrong if I did not admit I think in the cases where a lesbian couple have been abusing a male child that some of it stems from the very fact the child is male.
But since it is socially incorrect to even suggest abuse pathology is more likely to rise out of non married relationships. I imagine suggesting that additional factors may come into play in the cases of lesbian couples accused of abuse would be met with cries of homophobe and reminders that lesbians are gentle dolphin like creatures unlike those bruting hulks of mean ole nasty global warming culprits smelly men!!
I think you will find that a step parent or live in lover will abuse a child and that just because someone is a lesbian, that does not mean they have more of a proclivity to abuse a child than does someone who is not a lesbian or a gay man.