@Futureseeker it is very common with PTSD. It is even more common when a person has been sexually abused and raped.
If someone had forced you to do sexual acts, can you not imagine that it might put you off a little bit? It's not a matter of your wife having a lack of understanding, it's that to have sex with you, every touch might well bring an image of an abuser/rapist into her mind.
Can you imagine, if you were trying to enjoy sexual intamacy and somebody kept putting up pictures of atrocities in front of you, do you think you would be able to say 'oh well,' and continue to enjoy your sexual moment?
I'm asking those questions to try to help you imagine what your wife may experience. Trying to nag her into it, or explain to her that it won't be the same is way off with understanding PTSD. People with PTSD don't become stupid and unable to understand that other people have pleasurable sex, but they do re-live trauma as if it was happening right there and then, every touch, every emotion, every fearful thought that they are about to die. Sometimes it's not a flashback to that extent, but we get intrusive thoughts and images come into our minds. It is completely involuntarily, and those thoughts and images are of the trauma that happened. They can't be shut out.
If your wife is experiencing symptoms like this, then the way to help those symptoms get better is for her to go to regular therapy. This may take some time, and symptoms may become worse, so it may be that you would have to sacrifice your sex life while this went on. There are other ways of showing affection, and there are other ways of relieving your sexual desire too. It may also help for you to learn about what PTSD is, and to have some therapy for yourself to help you to cope.