Vader is too damaged to have actual sex, but he can use the Force to feel Aphra's sensations. So he makes sure that she has a lot of mind-blowing orgasms.

+ if Vader is always in his suit and mask
+ if he makes her come using his fingers
+ if he tells her that he expects nothing but compliance. And silence.
+ if Aphra masturbates while Vader watches
+ if they use sex toys
+ if some of these toys are hi-tech sex droids which they've designed together