An excessively fertile imagination
Apparently for Dr Johnson, women writing poetry enacted a "tribadic lust" by forcibly raping the female muse.
... I diagnose an overactive, and undersatisfied, imagination.
Complaints about the dangers of poetry for women go back a long way. In 1589 George Puttenham worried about the "poesies and devises of Ladies and Gentlewomen-makers, whom we would not have too precise poets lest with their shrewd wits, when they were married they might become a little too fantastical wives".
(From Subjectivity and Women's Poetry in Early Modern England, L. McGrath, Ashgate, 2002, Aldershot, p. 4)
But if you're still interested in Dr Johnson - and I have been looking forward to reading Norma Clarke's Dr Johnson's Women - I'd recommmend a visit to his house, which is just down the road from me. It is a remarkable survivor run on a volunteer basis - they need your support!