They do not talk about Utopia.
They talk around it with allusions and careful synonyms.
They leap to avoid it, and they stumble when they can't help it, but they catch themselves before they fall.
They dance around it. They're both excellent dancers.
They allude and they leap and they dance until they're both feeling very tired and very old and Jack can't seem to properly recall the face he had back then. He reaches up and strokes an unfamiliar cheek on a too familiar man.
"I hate myself for it."
The Doctor does not ask what this means. He just raises an eyebrow, but there are infinite universes in his eyes that speak far more than he says. "Just that?"
"No," Jack admits. "But more than the rest."
The Doctor nods. "Yup."
Jack doesn't move his hand for a very long time.