I have no idea why anyone could think its not a red card, the intent is completely and utterly irrelevant, dangerous play is, it was a flying kick, what if Arbeloa stoops only a bit to head the ball and Nani studs him in the face. Its insanely dangerous, he made full contact with his entire body weight behind the ball.
Its a combination of things, standing still and swinging a leg at a ball and you accidentally kick someone is no where near as dangerous as, running fast, jumping and sticking your leg out to catch someone with your full body weight behind it. These wouldn't even be comparable, accidental or not its dangerous play and a clear as day red card.
Personally I think on reflection that Colo's kick on Ba and Crouches kick on Reid are red cards because they were too dangerous, but both of those were a swinging leg, neither were lunges, neither had the full bodyweight behind the kick.
Either way, before and after the Nani red Utd had gotten the lead and decided to play for the 1-0 win and stay back, the game already turned, Real already had a different gameplan and were already flooding forwards and already looked far more likely to win. If Utd hadn't withdrawn so much after they got the lead they might well have won, even after being down to 10 men.