This didn't seem like a rape scene for the hell of it, just to amp up the shock value.

It was something that I'm thinking most of us had lurking in the back of our minds from the moment we learned she was going to marry this creep. It was one of the most obvious reasons why her decision to do so was scary and unsettling. And marrying him was her decision, and consumation (especially in that world) is a given part of that decision, so I'm not even sure rape is the proper term here. Traumatic and disgusting, absolutely. But that was the whole point of why we're like "fuck no" when the idea of her marrying him is brought up.

So it was very relevant to the story IMO, because it shows this is what she's willing to go through for whatever her reasons are. Its the very thing that makes that plotline so heavy.

Also, IF Reek ever happens to snap out of it and become Theon again or just a rebel-reek (and I have no idea if it will ever happen), I'm guessing that scene would play heavily into such a transition.