In fiction rape is a special crime because it can never be an heroic act. You can attack, steal from someone, or even kill someone and remain a sympathetic character under certain circumstances.
First off, I can't believe what I'm about to say here.
Have you ever heard of the Japanese adult RPG Words Worth or it's animated adaptation?
Yeah I'm about to present to you a situation from a Japanese erotic fantasy RPG where rape is not unquestionably evil. I don't believe it either. Fiction can be stranger than life. Or is this life being stranger than fiction in a weird meta way? Whatever.
Anyway, I'm not going to get too detailed into this because I barely remember much in the first place, but the action in question boiled down to a man attempting to fulfill a prophecy that would essentially save the world from catastrophe. The prophecy required the birth of a child bearing certain traits that would have been almost entirely unlikely to have been born outside of the rape he committed. So his situation was basically that he needed to rape to save the world.
You can contrive justification for just about anything in fiction.
If we want a different scenario (one that doesn't rely on prophecy), what about the classic "last man/woman pair on Earth" bit? If either party is unwilling is it justifiable to let the human race parish? I'm not looking for an answer, I'm just posing a question.
To be clear I don't believe rape is justifiable in any situation that's probable to occur in reality. You basically have to contrive a wildly unlikely fictional situation before you can even begin to start questioning whether or not rape could actually be an option. No one wants to do that though, because by most standards rape is so bad that you can't even propose a "what if" without the people around you looking at you like you were an actual rapist.
I thought about not posting this a couple times myself, but I think playing devil's advocate is important, and I trust no one here is actually going to think I'm doing anything but that.