Why cheat?

Within hours of Kristen Stewart's face being planted on every news tabloid imaginable with accusations of cheating, very public apologies were issued by herself and Rupert Sanders to their significant others.  This of course was a move planned out by publicists and certainly not for the benefit of those they cheated on.  The damage has of course already been done and by this point I'm fairly confident in stating that neither Rob Pattinson or Rupert's wife Liberty could care less which media outlet published such apology.  I know some people probably view the whole scenario as just another shining example of Hollywood romance down the tubes, but I cannot help but over-analyze the situation.