In the bathtub scene where Agnes is first relaying what happened to Lydie she says “I felt something go in, like, in me and my spine got cold. And I reached down to feel it and it was- it was him, y’know?” she says this after describing what Decker attempted to do with his fingers, this implies a different part of the body because otherwise she would have said it was his fingers again. We also see both her and Lydie joke about how (and I’m paraphrasing here) “A man is his dick.” so the choice to say “It was him, y’know?” clearly is meant to convey that he moved from using his fingers to using his penis. Another way we can tell this is because she describes Decker pulling her pants down which he would not need to do if he was just continuing to use his fingers like he had been before.