Lady Macbeth sees all things feminine as flawed. She appears to accept without question the notion that that which is feminine is weak and thinks that feminine qualities will interfere with her accomplishing the goals that she has set. (She does not seem to think that her ambition does not suit a woman). She tries to exorcise all "weak" or feminine qualities from both herself and her husband. This is not to say that neither she nor he have feminine qualities--she sees the feminine in both of them. Her fear of Macbeth is that he is "too full of the milk of human kindness to catch the nearest way". "Milk" itself is feminine and coupled with "human kindness", the image is one of a nurturer. She is afraid that Macbeth is too compassionate to kill Duncan. She has similar concerns with her own nature, hence her request to be "unsexed" and to have her own milk replaced with gall. Nurture? No. She wants to burn. Yet, for all of her talk, she remains, essentially female. She can't kill Duncan because he looks like her father and, belatedly, she feels such immense guilt at what she's done that her very soul is tormented. She is weak.
As for Macbeth, he seems assured of his manhood, yet is open to manipulation by Lady Macbeth. Whenever he refuses her or looks weak she insults his masculinity. He seems to know that he is fine--he tells her often that he dares only what a man would dare. Yet, after every speech, he does as she tells him. Even as she insults his masculinity, she teaches Macbeth to be feminine (if these qualities can be said to be feminine): he obeys her commands and he learns to dissemble while appearing to be fair (be the serpent under the flower).
Macbeth becomes a better strategist in his use of the feminine qualities he gleans from Lady Macbeth's instruction, but his true nature is not to hide what he thinks. Consequently, he reveals all his emotion when he is confronted with Banquo's ghost. His major flaw, however, is not his brush with femininity, but a surfeit of a masculine quality: over-confidence. Neither Macbeth nor Lady Macbeth can completely function as a blend of masculine and feminine, but for Lady Macbeth, the attempt is deadly.
It is fatal for Lady Macbeth to try to adorn herself in masculine characteristics; in the end she is too weak a vessel to hold the acid she wished to flow through her.