Have said this earlier too, in Aadi-Katha's past relationship they were both EQUAL partners, lovers, decision makers, pranksters & sufferers. In case of Viaan-Katha's current relationship its not the same.
Her yes, her choices, her decisions, her thoughts, her baggage have more weight-age than Viaan's, hence she is also expected to be more thoughtful, transparent & mature.
I will not justify what Katha did but comparing with her past relationship is wrong.
Aadi and Katha are college buddy, they were together for 4 year before getting into marriage. They were young and adaptive.
Viaan and Katha have only a month relation and may be some 6-7 month of knowing each other. 33 is quite a mature age and most of the people become rigid with their personality and not able to quickly change with the environment.
At young age it require lot of love and passion to maintain a relationship whereas in mature age it require lot of understanding and maturity to maintain the relationship.