We the viewers know the real reason behind the marriage, but the charcters on the show are oblivious. As far as they are concerned, what Arnav and Khushi did was a mutual agreement between the two.
So, to keep up with appearances, Arnav and Khushi may go on a honeymoon (with or without Payal/Akash). Then again, knowing Arnav/ASR, he might just make up an excuse like work as a way to get out of it.
What do you think?