never mistake that Dean turns around because future!dean wasn’t pointing at him. The camera fucking goes from blurry to clear and they show a Dean reaction shot. That is the exact moment when Dean found out that he and Cas in the future had a sexual relationship. He probably had high suspicions when he saw Cas but that (and the conversation that follows) seals it for him.
That is why he he says “send you friends to die…and Cas too” because he knew that Cas was and has always been more than a friend and he for sure knew that he was more than a friend to future!dean.
Oh My God, you’re right.