If the knight's experience of love and sex in "La Belle Dame Sans Merci" is so negative, that we wonder what it must that say about women? Nothing good. The fairy lady in the poem enslaves the knight and then ditches him, leaving him to mope around the lake "haggard" and "pale." And this isn't the first time she's done this. According to the knight's dream, there was a whole series of "pale kings," "princes," and "warriors" whom the fairy lady ensnared before the knight came along. Of course, the story is told from the point of view of the knight, and not of the lady, so it's possible that his perception is biased or inaccurate.
The motives of the lady in "La Belle Dame Sans Merci" are described only from the knight's point of view. Because the knight is not a reliable narrator, it is possible that the reason she weeps in her "elfin grot" (30) is that the knight has injured her in some way.
Although many feminist critics have read "La Belle Dame Sans Merci" as a pseudo-rape, the knight's ornamentation of the lady's "fragrant zone" in line 17 suggests that he is actually celebrating her femininity.