Duke knows the difference between good and evil.
He may have wobbly definitions of right and wrong, but he should be fine with good and evil.
Mara is evil, and Duke has been around the block enough that sex does not equal hypnosis.
(It's like you've never watched a James Bond movie.)
Just because they are sleeping together now, Mara might assume that Duke will follow her orders and do anything she says for as long as he thinks that more sex is on he table, otherwise she would not have taken her trousers off? Why manipulate the unmanipulatable? Simple. She cannot fathom a human being that she can not manipulate, so they don't exist. Don't get me wrong, she has done the right things to brainwash Duke, but I really don't think that he is falling for it, and he gains more to see how this plays out than laugh at her vaginal hubris.
Hmmm.
Do you watch Homeland?
Clare Dane's character is getting shit from all her subordinates because she had sex with a nineteen year old until he turned on his faith, his family and his militant political leanings, and then got shot in the face for allowing her vagina to turn his moral compas upside down.
(This is not sexist. Some men can control some women with sex too. If you can be controlled by sex, it doesn't mean that you are a man or a woman, it just means that you are an idiot, and being an idiot has nothing to do with gender.)