The authors primarily examined by Millett are from the age long after Romanticism: Henry Miller, D. H. Lawrence, Norman Mailer, and Jean Genet. Still, her critique of sexual politics in those authors can just as easily be retrospectively applied to earlier periods in the following examples.
Keats, who is cited briefly by Millett, extended his incomplete and unhappy love life into his verse in a mode that conveys an antagonism to women. In "La belle dame sans merci," the speaker regards himself and legions of men as victims of the beautiful lady:
I saw pale kings and princes too,
Pale warriors, death-pale were they all,
They cried "La belle dame sans merci
Thee hath in thrall !"
I saw their starved lips in the gloam,
With horrid warning gaped wide.....
Keats viewed himself as a victim of his frustrated love for Fanny Brawne. In addressing her, his tone is hysterical:
I cry your mercy--pity--love--aye, love!
Merciful love that tantalizes not.
As great as Keats is, his "blaming the woman" stance is unfortunately obvious here.
Byron—though in general his attitude to women is more mature—seems to repeat the trope of woman-as-antagonist in his greatest work, Don Juan. Each woman in Juan's life seems an obstacle, beginning with his interfering and obsessive mother, Donna Ines and his first lover, Donna Julia, and even his one true love, Haidee (though not through her fault, Juan almost gets himself killed by her father). Coleridge, too, in Christabel employs a related theme of a strange, ambiguous woman-seductress (Geraldine), though in this case the victim is another woman, Christabel.
One could say that the difference between these examples and the twentieth-century works Millett focuses on is that the Romantic poets saw women as exercising power over men and not the other way around. But it's really the other side of the same coin. In identifying women as antagonists, the Romantic male sees himself as pointing out his own alleged superiority. In Don Juan, the women characters are the ones actually victimized by men, not the other way around, and Byron does make this clear.
For women authors of the early nineteenth century, such as the Bronte sisters, the sexual dynamic is an uncomfortable one in which women are manipulated by men, as both Catherine and Isabella are by Heathcliff in Wuthering Heights and Jane is by Rochester in Jane Eyre.
Millett's point may be that far from becoming more progressive, the modern age has actually revealed male authors to perpetuate in a more obvious and naked form negative stereotypes of women and the issue of male power over women. The above examples from Byron, Keats, and Coleridge are mild compared to the portrayals in Lawrence and Henry Miller.