You're right about his body language and demeanour. Had it been a fling he would have tried everything to explain it to her. But it is not a fling and there's no way he could have said that to her without breaking her completely.
For me, I just felt his leaving was very abrupt. Both times, his mother asked him to leave and he just went. He did not even try to explain or put his side of things, not even to his mother. Infact, when I look back, not once did he try to reconcile, try to explain nor his body language gave that indication. It was as if he was already prepared and ready for the aftermath. I thought his silence was his acceptance of his crime but I think it was also a sign of his helplessness because of his feelings. They have shown us many times that he cannot lie, even yesterday and hence I guess he could not tell Mauli that is Nandini who holds his heart and not her so he was silent.I also like the fact he did not go immediately to Nandini which shows that Mauli devastation did have a impact on him and he is not an insensitive brute.Yes, I think during the whole confrontation as @mango_pudding said, Mauli was only thinking that it was fling, physical attraction, a mid-life crisis on his part and hence I guess she was ready to forgive him. I think the whole family thought that they will reconcile as if its a glitch in a marital life. Yes, Kunal has made a mistake but lets forgive and forget for the greater good. Even Mauli was coming to terms of it. I think if it was only phone call, the damage would have been Ok but the password was the last straw. It actually screamed at her that he had seriously replaced her in his heart. HIs leaving home was the final nail in the coffin. Yesterday, in true sense, she broke completely.Frankly, the more I see their reactions, the more I feel that we missed out on the path that CV's were taking. Like, I said before, it seems like the confession was not the start but the end and hence we did not see the guilt or those conflicting emotions at all on both of them. But, as you say, now that they have seen the impact of their actions, I want to hear that self talk or a conversation between them on the next steps and what they are thinking.I do not like Rajdeep's character at all but his dialogues do make me laugh. Mauli's mom is another type of person that I hate. She is the perfect reflection of all those women, who just do not want to change with time. Like, who blames your own daughter for your SIL misdeeds man. Its the attitude, right, someone is already broken, break them more and they take pleasure in it. I would have expected that from the MIL but here too CV's have shown the exact opposite which is a welcome change.Rajdeep is going to get that "Mahila morcha" to Nandini house. I hope this time they show her standing up for herself and not wait for Kunal to come and save her.PS: Also, I was reading that one of the sublines of this show is that "'Pyaar ek se zyada logo se ho sakta hai..par Aashiqui sirf ek hi se hoti hai'".I guess this is what they following and if I see the show from this perspective, leaving the EMA aside, it does make sense.