From the article:

“The reality of sex on screen makes many people feel awkward and uncomfortable, and not merely the actors. But what is it about sex – real sex – that so unsettles in film? When employed in the name of “art”, why does unsimulated sex burst the bubble we buy into when we watch a movie? And – if we can accept real sex is a cinematic turn-off – why is it used at all? …

…..real sex in films often serves an anti-aesthetic purpose, where the illusion of cinema is broken – we see all the floppy and pink bits. In short, the physicality of sex can be ugly. However, real sex on screen can be a transcendent experience, all the more beautiful for being real.

What is normal on many computer screens and a part of everyday life – indeed, fundamental to the continuance of life – becomes shocking when called art. Indeed, real sex often breaks the natural flow of fiction, disrupting our enjoyment of two otherwise pleasurable – or so one hopes – activities: having sex and watching a movie.

Given the power of unsimulated sex, it is little wonder that it has been used by filmmakers with an artistic pedigree to question moral norms, the nature of reality and the meaning of intimacy.”

Read more :

http://theconversation.com/von-triers
Mmmnn. In the hands of a skilled director, with the right sensibility, this supposed incongruity can be overcome. There is no need for explicit sex between actors in a feature film to be any less aesthetically pleasing or separate in any way from the overall visual storytelling of the film, just as real sex is a part of the normal fabric of life.

Amanda x