I use to be the same way and for me the reason was due to childhood sexual abuse... seeing the nakedness and how excited the men got in the movies left me feeling like I needed to run and hide for my own safety.

But I have since spent eight good years in therapy working on me and my own issues and now I can watch most nudity without feeling as though I am falling apart... I still hate drawn out rape scene and probably always will - its a female thing.