[DesertStorm]

Why do you think Nikita submitted when Tharun kissed her, She has witnessed Jay with Anjali and hurt about it all that time, Jay himself made a deal with Tharun to pursue her with out giving fight in her mind and she clearly established from these that Jay is not interested in her as a wife, lover and over years he did not make a move...so she wanted to explore other options...why would that be wrong? 

I have done my explanation about Nikita, you are not convinced, you will never be convinced.

~ RCF

RCF. 

I dont think the first two points had a bearing on Nikitha's mindset when she allowed that first kiss. 

She knows why Jay got into an affair with Anjali to protect their secret as some collegue saw them together 
(Later, she infact thanks Jay in her mind for his actions when Anjali is suspended and Sharmi talks to her, how Jay protected her reputation in the office)

Jay's deal with Tharun is also on the premise that Tharun would escalate Jay-Anjali relationship if he doesn't cooperate and he actually explains this to Nikitha and she playfully agrees to this in that chapter. Only during her confession with Tharun after accident she says their marriage died that day. I mean forget about whether she loved Jay in her mind or not. She playfully saying "So you sold your wife. Good that I am not your real wife"  and seeing his reaction of hurt and anger, she keeps continuing this saying I know how to keep him away from finding our secret etc.,

The other reasons like its a marriage of convenience that has an expiry date where Jay proving himself before his family, she never acknowledging/communicating her love for Jay or Jay not being expressive etc., would have definitely played a part. Jay even warns her that it is just lust/temporary fling for Tharun as he has a girlfriend whom he was planning to marry next year, so there is no future for Nikitha with Tharun.

Also, she never kissed Tharun herself until after the accident night before leaving to the hospital in the morning. Remaining all occasions Tarun initiates and she just goes with the flow despite setting boundaries and agreeing to take it slow. The kissing marathons in office where they set lunch time and evening time in bedroom attached to office didnt sit right with me as Tharun's status was not clear. I mean if it was only for sex, then they doing it is different but this is more like building intimacy with emotional connection while Tharun still has a girlfriend.