'The most erotic film of the silent era'

Excellent question! My guess is that it has something to do with the inversion of gender expectations*. Because the woman projects invulnerability and the man vulnerability, her mounting of the man while he's unconscious is redolent of the act of a succubus instead of a conventional lover. It's that undertone, I believe, which disturbed the male critic. Whether female critics of that era would be inclined to such associations is another matter. I'd imagine that female viewers would be less prone to reading the scene from the point of view of male anxieties and more attuned to the sense of spiritual union the scene evokes.

Of course, I'm just speculating about viewer reactions in the pre-feminist era. I expect most men today would not conjure up the image of a succubus while watching that scene.

*There is also something vaguely supernatural about a story of a man-spurning woman who lives alone in a woodland cottage.