I don't believe that people can fall out of love so easily. I come from a big Indian joint family where all my uncles including my Dad had love marriages and are still going strong. It's been so many years and all of the younger generation has grown up. But when we all go home on holidays the bond that is shared by each couples are so Adorable to watch. My dad still makes tea every Sunday for my mom. My uncle and aunty still go for jogging together and come back all smiles. They all still compliment each other on occasions and still irritate and tease each other. Yes they fight too. In fact they fight a lot. Even have disagreements. But they get over it. That's the key point of every relationship. You fight you disagree but you get over it and move on. My parents have always taught me one simple thing. If there is problem you talk. Yes you will disagree but at least you will know that what the problem is.
When Kunal started getting attracted towards Nandini the best solution would have been to spend more time with his wife. To talk to her, listen to her just talk about anything, go on a holiday no matter how small with her.
That would have reminded him how wrong he was to get get attracted to his wife's bff. And if nothing would have worked then he should have first talked with Mauli before anything else.
But... What can we say about our dear dear Dr. Kunal. A piece of work indeed.