I have had to edit your question to make it fit enotes guidelines, and because of that I have had to change the length. I hope it still reflects the original question that you wanted to ask. Well, what is key to realise in this episode as Winston finally goes to Room 101 is that this is O'Brien's last attempt to completely break and destroy Winston by ruining his hope and the rebellion that is represented by his relationship with Julia. Note what O'Brien says:
"By itself," he said, "pain is not always enough. There are occasions when a human being will stand out against pain, even to the point of death. But for everyone there is something unendurable - something that cannot be contemplated. courage and cowardice are not involved. If you are falling from a eight it is not cowardly to clutch at a rope. If you have come up from deep water it is not cowardly to fill your lungs with air. It is merely and instinct which cannot be destroyed. It is the same with the rats. For you, they are unendurable."
It is by using this method - Winston's greatest instinctive fear, that O'Brien is able to completely break Winston and any rebellious spirit he once had. Bearing this in mind, it is completely believable that Winston would wish this upon Julia - with our greatest fear essentially we are very selfish people and would rather anyone else but us would suffer it.