"At first it was all right. We lived sinlessly. We slept in the same bed, belly to belly sometimes, and o, I could feel the presence of the Serpent, but we. never. did. until. […] He… touched me. In that place. That woman place." (2.766)
Okay, there's a lot going on here. First, by Serpent, Momma means "man place" (you know, to go in her woman place). And by referring to her husband's penis as a serpent, she's making herself into a figurative Eve. So by having sex with him, she believes she committed the Original Sin. And guess what lucky girl gets to be the product of that sin? Poor little Carrie.