By itself, this line may not seem to be exactly dripping with femininity, but the line is repeated almost exactly five lines later. The repetition of the lines gives the distinct impression that this faint, sweet voice is very much a part of womanhood, which itself is closely tied to innocence and being utterly harmless. What Coleridge seems to be getting at here is that, though perceived as innocent and benign with a soft voice, women can be just as dangerous as men—perhaps even more so in some cases.
Me, even me, a maid forlorn; They choked my cries with force and fright, (82-83)
Geraldine is playing up the innocence here, acting shocked that even she would be treated with such brutality.
Since one, the tallest of the five, Took me from the palfrey's back, A weary woman, scarce alive. Some muttered words his comrades spoke: He placed me underneath this oak; He swore they would return with haste; (93-98)
At this point, a modern woman might wonder why Geraldine didn't just get up and leave. That may be Coleridge's point though, that women were so used to following orders that they often didn't even understand that there might be more options available—much like a flea in a jar.
Then drawing in her breath aloud, Like one that shuddered, she unbound The cincture from beneath her breast: Her silken robe, and inner vest, Dropt to her feet, and full in view, Behold! her bosom and half her side— A sight to dream of, not to tell! O shield her! shield sweet Christabel! (247-254)
Coleridge seems to imply here that a naked female body is so bewitching and overwhelming that even another woman needs prayers to overcome it.
Yet he, who saw this Geraldine, Had deemed her sure a thing divine: Such sorrow with such grace she blended, (475-477)
Sir Leoline is powerless in the face of such beauty, making Geraldine even more powerful than a knighted man (and therefore even more dangerous).
The lady fell, and clasped his knees, Her face upraised, her eyes o'erflowing; (519-520)
Here we get a classic example of a woman using her most convincing feminine wiles.