That dance in the bar...

I was wondering the same thing! lol

I came here to look for this! Turns out it's just for the movie. I found an interview:

http://flavorwire.com/547399/simon-pegg-and-lake-bell-on-subverting-rom-com-conventions-and-dancing-to-duran-duran-in-man-up

one of its most memorable sequences finds the duo on the dance floor, conducting an argument about Pegg’s failed marriage in the midst of an impressively choreographed dance-off to Duran Duran’s “The Reflex.” “It was just this idea that Tess had, that we all have a dance routine somewhere that we learned unconsciously as youngsters in the discos,” Pegg says, “and that Jack and Nancy, whilst they’re arguing, find this odd synchronization, which kind of hints at the fact that they are very much in tune with each other. It is one of those ‘that wouldn’t happen’ moments, but it worked figuratively in the movie as a way of saying ‘Look, they’re fighting and they’re arguing, but they’re totally in tune with each other.’”