I waited all this time for @apoorvm to come back and say something. I mean if his quarry is well replied.
Wonderful explanation shared by @DivineSoul Thank you. You have given a very detailed and wise information.
Puneet is my lovely young brother, who pushed me on top of the Grapevine by prefixing "great" before my name. All glories goes to Sharat Sir, who deserves the appreciation. I am only his student. Besides, Puneet has put in another angle to the discussion. Great effort Puneet bhai.
Although, much is already explained, I would like to summarize it in short. Please understand, Material & Spiritual worlds are totally different. Sometimes called as Maya, Material world is vulnerable to both positive & negative, Spiritual world, on the other hand is Crystal clear and pious. Junk food or porno, both are material, and what is material cannot touch spiritual modes directly. But material things may always pursue a spiritual degradation. Let me opine. Material inputs affect spiritual avenues on the Mental levels. Our mind is the junction which translates and transforms our "Bhava" or emotions/thoughts. This level is available to be approached by both – Material and Spiritual. Now, on one side, we have Material things – Junk food & Porno which is governed by the senses ie. Taste & Lust and on the other side is the Spiritual levels which is governed by Serenity and Purity. One is Excitement & Agitation, while the other is Calmness & Peace. Being a disparity of the opposite kind, both cannot exist in the same state of mind. I mean, one has to give space to the other. So once overpowered by lust, mind may channel the spiritual levels downwards. This is the science in this issue.
Now, what should really be analysed is the reason, @apoorvm has asked this question. But Let me first be clear that we really do not have any grudge on anyone's personal preference/behavior, and this should be taken in No offence. Be it for ourselves or for someone else, what drives us to ask for preservation of sanctity, is our attraction towards a specific arena and our doubt that whether our indulgence in it, is proper or improper. I will call it doubt and not guilt because if justified by our consciousness, nothing in this world is wrong. Only a pinch of doubt makes it a Sin. This, however, does not makes a bad karma good. Ratnakar was a Robber but he robbed and killed, considering this activity as livelihood for him and his family. That is why, when the time came for his transformation, he was given the opportunity to become one of the greatest Sage of Vedic times – Valmiki.
If the ideology of ISCKON has to be believed, we may see that devotees who go for Sankirtan (musical gratification of the Lord), will not accept cooked food from the house they have gone for the program. Rather they may happily accept Milk or Fruits from them. It is because, they believe that cooked food bear the thoughts and emotion of the person who cooked the food. So they take food cooked by a devotee only. What does this thought convey? If the Vaishnava ideology is to be believed, then it indicates that Food do pass energy in variance affecting our mind. Dear @apoorvm, in my opinion, now-a-days one cannot be so categorical in eating habits. Please carry on, Junk food does not affect your Spirituality at all. But please try to have junk food in limits. Please invite me sometimes.
Referring to the Lust part, living in this world, one cannot be detached from senses. In Vedic times, even great sage Vishwamitra fell victim to it. But take it from me, if you have a Spiritual inclination, your senses will obey you. They will not overpower you. You have to board the bus and you can board one. Control the naughty sense and elevate, its more interesting above and beyond.