Well, since you're trapped in your marriage anyway and cannot get a divorce, I suggest you start talking to your husband. Sumit is happily married, yet he's sexually deprived and you're emotionally deprived. Two wrongs don't make it right.
You are using Sumit to unload your unhappy life, now Sumit is trying to use you for sexual favors. Again two wrongs don't make it right.
I can almost guarantee you that if you give in to his sexual favors, your asking for trouble. If your husband finds out, he probably will end your marriage or make your life even more miserable.
The only solution to your dilemma is that you go to couple counseling and if this is something you don't do in your culture, you should start talking to your husband. Maybe you want to tell him the things you tell Sumit and maybe he'll surprise you with his own confession of unhappiness. You are married for better or worse and since divorce is not an option, you surely don't want to continue living like this. So start talking to your husband, give your marriage a chance.