What's the difference between eating food and adultery?

(Below is just my personal note, not the clear answer).

Shame to blame females where i have billions of children to take care (body cells).

Shame to praise males when i am unable to understand above.

Shame to blame householders when i am a leader and dweller among billions of houses(cells).

Shame to praise meditation practitioners when they are too not free from above.

Shame to praise 1 time eater when crave for food is still there.

Shame to blame many times eater when above ones also crave like this one.

Then what could be right food & how it works..?

When a person dwells into world of continuously birth. & death, a world where that person is leader then before starting to get rid of that world, a knowledge of 'I' has to be obtained. This 'I' that feels like dwelling and can also say to be troubled by defilements is in nothing more than the "thought process affected by defilements". So, see these defilements.

Then how will one see these defilements, if 'I' is also defilement..?

Simple, by feeling and realizing defilements as 'I' instead of my(which gives feeling of separate from I). This practice makes one aware to the level of seeing the actual difference between past and present behaviour and also one might be able to realize that 'Mara' is also this 'I'. Not only this one rather slowly-slowly this practice would enable one to see "how imagination, images(real or fake) started from self-image and ending to self-image" i.e. how karma(physical and mental) starts from one and returns to one.

If 'I' am mara then how to free..?

No need to do anything else. This special practice "will" start dimnishing defilements and later one can clearly see when past removed defilements return and can also see the images of these defilements as separate from current-self-image. Not only this, boy can clearly see that girl whom he is wishing to have is self-image in brain.

But how can defilements go so easily, these are from those infinite moments from past...?

Obviously one will be confirmed of 'mara' only when these defilements would keep on arising even when person is 'no longer interested' that's the point of test. Breath shall help, when breath is also of no help then separated image shall help, when it would fail then determination "will" help.

So, after achieving that knowledge, determination is the one that must be strong. There are chances that determination will be shakened then past memory of achieving calmness will help.

What are other effects of food?

Food taken in quantity more than required to work enthusiastic must be abandoned. Because with more food, more population increases. That includes more energy exchange within one's country(body), the holes(phassa) arise more due to that and hence makes one sensitive to gain and loss.

So, for meditators and busy ones:: quantity must be chosen by them such that they can feel light, stable & balanced during night and morning even without morning poo. Till that state, food quantity to be varied and tested.

Food affects reproduction, population, reason to sensation and back from sensation to re-assembling(satth-aayatana).

There's Buddhist doctrine on that subject in the Bhikkhuni Sutta (AN 4.159):

'This body, sister, comes into being through food. And yet it is by relying on food that food is to be abandoned.' Thus was it said. And in reference to what was it said? There is the case, sister, where a monk, considering it thoughtfully, takes food — not playfully, nor for intoxication, nor for putting on bulk, nor for beautification — but simply for the survival & continuance of this body, for ending its afflictions, for the support of the holy life, [thinking,] 'Thus will I destroy old feelings [of hunger] and not create new feelings [from overeating]. I will maintain myself, be blameless, & live in comfort.' Then he eventually abandons food, having relied on food. 'This body, sister, comes into being through food. And yet it is by relying on food that food is to be abandoned.' Thus was it said, and in reference to this was it said.

... and ...

This body comes into being through sexual intercourse. Sexual intercourse is to be abandoned. With regard to sexual intercourse, the Buddha declares the cutting off of the bridge.

In other words, food is one of the necessities or requisites of "the holy life" -- and sexual intercourse is not that kind of necessity.

See also the topic Why the Buddha abandoned asceticism which has more on the subject of not-eating.

i am trying to understand how "eventually" occurs

I think the Theravada doctrine is that enlightenment happens in stages -- see Four stages of enlightenment.

So the first stage is to be free of identity view, and to see the dhamma correctly, and so on.

The two stages after that are minimising and (eventually) being free of sensual desire and ill will.

And the last stage etc.

how "feelings destroy"

I'm not sure I understand this question of yours.

I think that the doctrine about "feelings" is summarised in the doctrine of the 12 nidanas, which is that feelings and delighting in feelings leads to craving (for the delightful feeling to continue or to reoccur), etc.

There's also doctrine related to the "three poisons" -- i.e. that pleasant feelings tend towards craving, unpleasant feelings aversion, and neutral feelings ignorance or confusion.

Hence the advice about food: don't abstain from eating in order to create a feeling of hunger, don't over-eat to create a feeling of overeating, just eat enough to remove the feeling of hunger -- the "middle way", neither one extreme nor the other.

if not "i" is there then "why need to support further 'i' by worldly activities & how does this happen"

You might want to read the various topics tagged suicide, I think they say that suicide is not the right way to liberation from egocentricity and self-view.

Read the four noble truths again:

1. death is painful, birth is painful, etc.
2. suffering is caused by craving -- which includes the craving for becoming, also the craving for cessation

I think that Buddhist doctrine says that human life is rare and valuable -- a rare opportunity to meet and understand and practice the dhamma, and to thus experience liberation and unbinding -- which is not the same as the wheel of life, the cycle of death and birth.

2nd para & your conclusion:: i nowhere asked for "sexual intercourse"

Well it's the definition of adultery

voluntary sexual intercourse between a married person and a person who is not their spouse

Perhaps you said "adultery" as a metaphor or something, not meant literally. Even so, if the advice about intercourse isn't relevant to your question, then the advice about food might be.

Also if your thoughts about "the body" (e.g. cellular division) is causing thoughts to tend toward suicide, SN 54.9 might be relevant -- the Buddha set a group of monks to meditate with "foulness of the body" as a meditation subject, after/during which some monks killed themselves ... after which the Buddha taught a different kind of meditation, related to breathing ("anapanasati").

I think that the "meditation on the foulness of the body" type of meditation is recommended only for people who have a trouble with lust, i.e. as an antidote to sexual desire or desire for adultery -- and isn't meant to suggest that suicide is the correct solution.

What's the difference between eating food and adultery?

Big difference. Without food, you'll die and completely miss the rarest chance to cultivate the Path in this human form. You will not die due to not having adultery. And not only that you completely miss the rarest chance to cultivate the Path, you'd completely wreck it. Matter of fact, engaging in adultery and you're already "dead", and will continue to be dead countless times throughout this endless Samsara.

Perhaps you are confusing the word "adultery" with the words "reproduction" and "sex".

When you eat food, you are definitely supplying energy to the body and giving it the necessary ingredients for cells to reproduce.

The cells in your body do not have sex to reproduce. Sex by the way, is not against the five precepts, if practised within the context of a committed exclusive monogamous relationship by lay persons.

Sex however, is not part of the monastic life, although food is. ChrisW's answer addresses this perfectly. Food is needed to sustain the body and you need to consume food to survive, but sex is not needed for sustenance - hence a monk needs to eat food, but does not require sex.

The subject of food for the sustenance of a person seeking freedom from suffering, is well discussed in SN 12.63 (quoted below):

"And how is physical food to be regarded? Suppose a couple, husband & wife, taking meager provisions, were to travel through a desert. With them would be their only baby son, dear & appealing. Then the meager provisions of the couple going through the desert would be used up & depleted while there was still a stretch of the desert yet to be crossed. The thought would occur to them, 'Our meager provisions are used up & depleted while there is still a stretch of this desert yet to be crossed. What if we were to kill this only baby son of ours, dear & appealing, and make dried meat & jerky. That way — chewing on the flesh of our son — at least the two of us would make it through this desert. Otherwise, all three of us would perish.' So they would kill their only baby son, loved & endearing, and make dried meat & jerky. Chewing on the flesh of their son, they would make it through the desert. While eating the flesh of their only son, they would beat their breasts, [crying,] 'Where have you gone, our only baby son? Where have you gone, our only baby son?' Now what do you think, monks: Would that couple eat that food playfully or for intoxication, or for putting on bulk, or for beautification?"

"No, lord."

"Wouldn't they eat that food simply for the sake of making it through that desert?"

"Yes, lord."

"In the same way, I tell you, is the nutriment of physical food to be regarded. When physical food is comprehended, passion for the five strings of sensuality is comprehended. When passion for the five strings of sensuality is comprehended, there is no fetter bound by which a disciple of the noble ones would come back again to this world.

"Adultery" and "sex" does not mean the same thing either. Adultery is sex outside of a committed exclusive monogamous relationship, and this violates the precept on sexual (or sensual) misconduct.

What is natural and unnatural? Is food natural? Yes. Is sex natural? Yes.

Then doesn't the Buddha teach what is natural? Actually, the answer is no.

The idea of the self, craving, clinging, sensual pleasures, sexual attraction, passion of becoming, ignorance - all these are natural. But they lead to suffering and perpetuate suffering.

The Buddha realized after enlightenment that it's hard for him to teach people to go against their nature, which is what he said in SN 6.1 (quoted below):

"This Dhamma that I have attained is deep, hard to see, hard to realize, peaceful, refined, beyond the scope of conjecture, subtle, to-be-experienced by the wise. But this generation delights in attachment, is excited by attachment, enjoys attachment. For a generation delighting in attachment, excited by attachment, enjoying attachment, this/that conditionality and dependent co-arising are hard to see. This state, too, is hard to see: the resolution of all fabrications, the relinquishment of all acquisitions, the ending of craving; dispassion; cessation; Unbinding. And if I were to teach the Dhamma and if others would not understand me, that would be tiresome for me, troublesome for me."

However, in Iti 109 (quoted below), the Buddha indeed teaches man to swim against his nature to become free from suffering. Renunciation is against the flow i.e. it's not natural to man.

This was said by the Blessed One, said by the Arahant, so I have heard: "Suppose a man was being carried along by the flow of a river, lovely & alluring. And then another man with good eyesight, standing on the bank, on seeing him would say: 'My good man, even though you are being carried along by the flow of a river, lovely & alluring, further down from here is a pool with waves & whirlpools, with monsters & demons. On reaching that pool you will suffer death or death-like pain.' Then the first man, on hearing the words of the second man, would make an effort with his hands & feet to go against the flow.

"I have given you this simile to illustrate a meaning. The meaning is this: the flow of the river stands for craving. Lovely & alluring stands for the six internal sense-media. The pool further down stands for the five lower fetters. The waves stand for anger & distress. The whirlpools stand for the five strings of sensuality. The monsters & demons stand for the opposite sex. Against the flow stands for renunciation. Making an effort with hands & feet stands for the arousing of persistence. The man with good eyesight standing on the bank stands for the Tathagata, worthy & rightly self-awakened."