Where is the line and what is 'crossing the line'

I was pondering over one of the thoughts for quite sometime now and I would like to start that by asking - Why is having sex outside an existing marriage considered wrong or why only that??
PS: 'wrong' is a very wrong word, but for want of a better word let me continue to use it.

For more clarity let me explain - say a typical marriage, working husband Bunty, housewife Babli...

• they both are very much in love...in the beginning
• they are inseparable...in the beginning
• they eat together, drink together and have a lot of sex...in the beginning off course
• Babli likes Sharukh, they only watch Sharukhs movies...in the beginning
• Babli likes zee tv...they only watch zee tv
• fighting, arguments? no never...it is still in the beginning

While all this is happening TIME is a very cruel observer...he comes in between them...he catches both their hands and walks them through each others hidden rooms...
....when I say hidden room, this is the theory - I think all of us have 2 rooms within us; one front room, which is clean, and this is where we meet people in the beginning, and two is our dirty room, where we stack all our not so nice stuff and all that, which we generally don't want people to see, but few of our very close friends and family members would have walked into and lost the awe we posed to have (no they dont hate us, but the initial hype about the personality is lost). During childhood I would be wondering how great it would be if my father was an Amitabh Bachchan or a Gandhi??.. while whole world watch them on tv or news papers wouldn't it great to watch them day and night and also what not?? but you ask Abhishek, maybe he doesn't think so much more about his father than I think about mine...that's because, our common enemy - Time had helped him see his fathers hidden room.

...now back to our Bunti, Babli - Time was busy showing nooks and corners of back rooms... now, eek! Babli comes to know - Bunty doesn't brush his teeth before he has his morning coffee, he was doing this all his life, why is it so eek now??!...the differences start, arguments begin, fights happen and all that.. what the hell, let me brush the teeth...Bunty yields, Babli conquers...this is the beginning of "change Bunty movement" and mind you change is not easy...monkeys changed to human beings only in a zillion years and I am sure Babli's ancesstors were involved even it that!

with all this changes, etc...the friendliness is slowly lost between Bunti and Babli, he starts getting busy at work, Babli is more cranky, encounters are kept to minimum to manage tension.

At this stage, their inseparable lives started becoming separate...once upon a time when they used to always go together, eat together, watch tv and movies together, they start doing them separately...The other day Bunty was mentioning of a second tv and Babli shot him down!

Hmm! they still have sex ...but now a days rarely...not that drive has come down, but it is difficult to find time in between quarrels!

One of these days, Bunty meets his new colleague Maya...good looking, smart and all that, so Bunty quickly cleans his front room (where he introduced himself to Babli earlier) and they both kept meeting there...firstly work related...but slowly Maya gets interested in Buntys personality, our man - Bunty is smart and witty and Maya used to laugh at all his jokes (Babli had stopped doing that long back, she thought they were silly) they start going out, for lunch, dinner, movies...

and one fine day they crossed the line...they had sex!!!

Note: In the above Bunty and Maya story it could have been Babli and say neighborhood Rohit (Bunty was so busy at work, Rohit entertained Babli at home!) also...

now coming back to my original question - why is having sex outside an existing marriage considered wrong or why only that??

...why did I have to say crossed the line when they had sex, why not when they met, laughed, had fun and all that??
... Let me even change the premise a little bit. Let it not be Maya also, say Bunty's childhood friend Ramu relocates to Bunty's town and they start meeting regularly and have booze and have night long chatting sessions and ignored Babli all together... why is this not crossing the line??

If any of the partner has fun while the other is not part of that, should that be crossing the line?? or otherwise just for the sake of discussion - should fun be a free domain - Bunty, Babli, Maya, Ramu and Rohit can all have fun the way they want it ??

How to pursue personal passions?

Last few weeks I have witnessed some situations, which led to some of these questions-

What happens if you are passionate about doing or becoming something in life and that is exactly opposite to your partner or your parents thinking? It becomes easy if one direction is a good and other is bad...but life is not such black or white, sometimes, they both are good in their own way. How do we solve the problem?

Modern way of thinking suggests each and every individual has complete right on what he does and so the younger crowd may end up thinking I am talking nonsense, it actually very simple, his life, his passion gets preference. But is life so isolated, are different lives not connected and affect each other??

Say its a one child family, mother and fathers lives are revolving around that child (Indian reader can easily identify with this), child grows up and becomes an individual of his own. He decides, he wants to become a ascetic (a Sanyasi)...now even his parents regard other ascetics and they have themselves thought this child to respect and revere them, but now...they are disturbed, shocked, hurt and what not beyond any explanation.
The child, now a young man is choosing something they all revered, so that is not bad, but parents want to see their son, progress in material life, get married, have children and all that...and also pray and revere somebody else who is a Sanyasi, swami! I think they are following the common man philosophy, most of us lead that life, so even this is not bad.

Applying modern thinking, the young man should do what he wants to do, well, then the mother is very sad, she gives up eating regularly, falls ill, etc...seeing this the father is also unhappy. With all this their child is also unhappy. Becoming a Sanyasi he should be a source of happiness if not all the Mukthi and all that, which we are however not clear. I think Happiness is to some extent in the direction of Mukthi.

So because parents are unhappy, does he sacrifice his passion (or if passion seems odd with a Sanyasi; dispassion)? Even then the unhappiness flows the opposite direction...from the young man to the parents.

What is the solution for this dead lock?