He was trying to rationalize the things for Shristi and in the process he was consoling himself. When you are a passive spectater, that's the only way you rest in peace. What a fairy should Pratima be, Arun still dares to think that lust and love can exist together in pure form and blames thoughts to be impure. Hypocrite, isn't he? So called civilization allows sex orgy until your bachelor party and expects you to be monogamous next day onward. I think no matter how strong the bond of love is, it can not bind you from provocative temptation.