Very nice post .I think more than what the secret is,he would be more disturbed by the fact that there is a SECRET side of his Nakku which was hidden for him and his love did not trust him enough to tell him.He has always closed himself in a shell away from the world because he thinks no one can be trusted and he did not want to get hurt and betrayed again.After loooong and reluctantly he has opened himself in front of Nakku and made himself vulnerable again.He considers Nakku utmost honest and truthful.Never can he imagine she would have a side which is hidden from him...it would reignite his trust issues.And hot headed he is,so his logical mind would shut off for some time and his dark side would take over where he would be clouded by demons of his mind .And it would not help that FR would be presented to him in a overdramatic way by evil trio,dipped in venom .
The phrases like "uspe kabhi bharosa mat karna " (DP),Jab tujhe uska suspense pata chalega or something like that by Anna ,Even Ganpat saying ye maan beti apko dokha de re hai...this simple truth would be contorted and deformed in a vicious way before he can clearly see the reality..and then only after long he would be able to come to a point to say...Oh! big deal,its just skin colour..and has nothing to do with dhokha