Love has blinded the poet's judgment, making him see the Dark Lady as beautiful when his eyes and heart know she is ugly by every standard—a willing embrace of delusion.
Her sexuality is a harbor where all ships anchor. His heart is anchored to a faithless woman.
She is a 'several plot' (exclusive, private property) but he knows she's 'common place' (a common prostitute, owned by many). He chooses the lie of exclusivity.
The sonnet uses imprisonment metaphors throughout: eyes 'anchored' in the bay, heart's 'judgement' is 'tied' by hooks. The poet is held captive not by the Dark Lady directly but by his own corrupted sensory apparatus. He can see what is true (she is not beautiful, her face is foul) yet simultaneously sees her as beautiful. This duplicity of vision is the poem's central horror—not that he is deceived but that he is complicit in the deception.
The couplet—'In things right true my heart and eyes have erred, / And to this false plague are they now transferred'—suggests that the poet has become emotionally infected. The false perception is a contagion, a 'plague' spreading from the Dark Lady to his sensory organs. Yet he remains infected willingly. The poem is one of Shakespeare's most powerful descriptions of how desire warps consciousness.
Like knowing objectively that someone is bad for you but finding them attractive anyway, and hating yourself for it. The poem captures the self-aware lover who cannot trust his own perception because desire has colonized his judgment. He sees the truth and chooses the lie.