I would disagree about there being ambiguity as to whether Tess was raped. I think this question misunderstands the actual debate about that specific scene.
Let's take a look at the scene in question
There was no answer. The obscurity was now so great that he could see absolutely nothing but a pale nebulousness at his feet, which represented the white muslin figure he had left upon the dead leaves. Everything else was blackness alike. D'Urberville stooped; and heard a gentle regular breathing. He knelt and bent lower, till her breath warmed his face, and in a moment his cheek was in contact with hers. She was sleeping soundly, and upon her eyelashes there lingered tears.
Tess is asleep. Tess can not give consent. Consent means that you give permission for something to happen. Tess can not give permission because she is literally unconscious.
The ambiguity in this scene isn't about whether Tess was raped. The ambiguity stems from how the scene was written. After Hardy tells us that Tess is asleep, the narrative essentially ends. The next chapter begins "some few weeks subsequent to the night ride in The Chase". The question of why this gap occurs, and what affect the gap has on the story, is a very interesting question best addressed in a followup question.
Another interesting question is about what this scene tells us about gender and class. Hardy is clearly trying to make some sort of criticism of the dynamics in this scene: in the next paragraph, he essentially asks why this scene occurs over and over again.
Why it was that upon this beautiful feminine tissue, sensitive as gossamer, and practically blank as snow as yet, there should have been traced such a coarse pattern as it was doomed to receive; why so often the coarse appropriates the finer thus, the wrong man the woman, the wrong woman the man, many thousand years of analytical philosophy have failed to explain to our sense of order.
There's a lot you can critique about Hardy's portrayal of this scene. For example, the passage seems to imply that we should give up trying to stop sexual assault. The sentence "many thousand years of analytical philosophy have failed to explain to our sense of order" implies that sexual assault is a strange, unexplainable phenomena that can never be stopped. Another interesting followup question would be to ask about the political message this passage conveys about rape, class, and gender.
There are a lot of interesting questions about this passage where there's a lot of room for debate. I don't see how there can be any debate about whether Tess was raped, given what actually happened.