There is a reason Foster has used menstrual cups for years. Pads and tampons absorb his blood, wasting it, depriving him of his need, stealing it from him; with a cup, he is free to succumb to his ugly desires, his disgusting compulsion to touch his own filth and essence at once. There's a special kind of sick indulgence in it--by touching his own contaminating fluids, touching himself, he effectively wallows in the filthiness even as his body purges it.

Seeing her dispose of it so carelessly--

He feels that familiar edge, the muted hysteria of not knowing whether he should laugh or cry. Of course, he's incapable of either, so he just watches helplessly, feeling a little wrong and yet... right.

What pushes him over in the end is the opening between his legs, his bloody needing gash that he cannot touch--the sight of it, stained with blood, is more than he can stand. Her fingers seek that needy opening, pressing inside, and he releases the breath from his lungs in a kind of desperate, miserable want for relief, her warming, probing fingers unable to curb his urgent greed, his craving--to touch it, to feel it, to feel himself.