I guess it means that she believes that sex is better now that he's gone, and he tries to explain it saying that "normally she never feels a thing". It's like this: I'm the man -and i have my pride of course- so i don't accept very well that she can actually enjoy sex -because she didn't enjoy it with me "she never feels anything"- so, i as a hurt man say that she's insensitive. On the other side she says that she's a lot more satisfied with sex by herself than how it was with him, so, is not that she doesn't know how to feel, but that he didn't know how to make her feel. That's my interpretation. there's where it leads me with a little more sense.