It's not inherently offensive. It can be, but that's with the intent, not the word. Obviously when using it as a threat against someone such as "I'm going to rape you" or "I hope you get raped" then yeah it is offensive. Then again, saying something like "Shit, I just got raped in that match" or "Did you see the new Star Wars re-release? Why does George Lucas ave to rape my childhood" aren't really offensive, ignorant perhaps, but definitely not offensive. I think it's really the difference between targeting yourself and targeting others that defines the offense, mostly because targeting to yourself doesn't imply you're trying to ruin someone else's day.
The issue I have with the PTSD comparison is, while I'm sure the above mentioned threats would most likely trigger a reaction in a survivor, I'm not convinced that the word by itself carries that connotation. Not saying that it's never happened, but I get the feeling that a person who has a reaction to the mere mention of the word probably isn't handling it the right way. To me it sounds too much like a person trying to run away from the issue, which isn't wrong to do but won't solve things in the long-run.
The issue I have with the PTSD comparison is, while I'm sure the above mentioned threats would most likely trigger a reaction in a survivor, I'm not convinced that the word by itself carries that connotation. Not saying that it's never happened, but I get the feeling that a person who has a reaction to the mere mention of the word probably isn't handling it the right way. To me it sounds too much like a person trying to run away from the issue, which isn't wrong to do but won't solve things in the long-run.