But as far as I’m aware there are no rules against showboating or being disrespectful to your opponent in the ring. Otherwise Ali, Roy Jones, Prince Naseem would have been fined up the wazoo. Though an argument can be made distinguishing humping from dancing and taunting your opponent, the effect is essentially the same, in that no rule is violated.
FYI, Maidana got him back and did more or less the same thing to him on round 11 (I think). And no, this is not a common occurrence in boxing.
I seem to remember Joe Calzaghe doing this to Bernard Hopkins some years ago. Bernard got his with a slight low blow but really exaggerated the effects hoping to get a point deducted from Joe. So I took it to mean Hopkins was acting like a girl (a girl that got punched in the nuts)