This is a sensitive topic.. only for the reason that everyone is projecting their value systems, morals, thinking while analyzing this and this is but natural!!!
But just terminology wise we need to get something straight!!
What Shyam and Arnav's dad did was attempt to have affairs outside their marriage. This means inspite of being in a committed relationship... they tried to have a relartionship outside of that. This makes them cheaters and what they were trying to have with the other woman, here in question being Garima and Khushi would be labelled an extra marital affair!!
So Arnav cannot be judged on that same scale!!
If he accused Khushi of being characterless (which i agree was the harshest thing he has said to her) yet it was based on his misconception that she was having a relationship with a 'MARRIED MAN', which makes her look like a home breaker!! He didnt call her characterless because she was in a relationship before marriage... hell he was seeing Lavanaya too, so how can he view being in a relationship as characterless.
I know there are many people here.. who believe in the one love for life and married to that person concept but it doesnt happen like that all the time.
People do have failed relationships for various relations and that doesnt mean they are not allowed to move on and fall in love and get married!!
Like I said the reactions are because of the difference in value systems... but if we just accept the reality of what we see around us and view it not personally but objectively.. we may be able to understand this better!!!