"arousal can be a response to fear."
this is a page dealing with male rape.
men raped by other men often become aroused during the rape, and guess what, that alone leads to terrific trauma because straight men being raped by a gay man is a very real thing and being aroused by rape of yourself must be hard to deal with in the long run.
http://www.vslambeth.org.uk/rape%20men.htm