Q:

How do I describe an accidental kiss between the two main characters that is romantic as well as regretful?

How do I describe an accidental kiss between the two main characters?  
I want to make it so they got caught up in the situation and that both of them apologise to each other after that, like a kiss that is supposed to be romantic as well as regretful. 

A:

Speaking as an American, I think this is a difficult task in the current culture. Your best bet is likely a setting where the characters are a bit drunk, and the woman initiates the kiss.
That will avoid many of the politics of men sexually harassing women, if he does not initiate the sexual contact. (politics with which I agree.)
I'd suggest a party of some sort; a woman with a reason to over-indulge (a recent breakup or other emotional trauma). A man she has had a non-romantic or only slightly flirty relationship with for some time; a man that considers her a friend. Put him in a position of helping her, or taking care of her; I can imagine she slips, he helps her up, in this non-romantic embrace of getting her to her feet, she kisses him and he responds in kind; but breaks it off: He feels guilty for taking advantage of her state. Perhaps he leaves, or brings her home (but even there has honor and refrains from further contact because she is drunk, and he's not "that guy").
Proceed from there; kiss accomplished, they both remember it. They can be in love and denial of it, at that point.

A:

I kissed a close friend of my wife by mistake last year. She was mortified and I must admit that I had mixed feelings of amusement and embarrassment too. But wait - before you go running to tell my wife I should tell you that I already confessed. And it was alright.
Here's how it happened. The friend of my wife is fond of kissing on the cheek in the Continental European style - don't ask me how she got that way - she's English through and through and we don't usually go in for overt displays of affection like that. Usually. Anyway, she goes in to kiss me on the left cheek and I go for her right cheek at the same time and (yes, you guessed it) we met in the middle - lip to lip.
Nothing romantic to it. Purely accidental and a little (but not much) regretful.
Anyway, moving on: I just looked up this on TripAdvisor about Egypt:

Affection: There will be some young couples holding hands here and
  there but any further public display of affection is not the norm. No
  kissing in public. Like other Muslim countries, men can be friendly
  towards other men and women can be friendly towards other women. But
  women, especially non-family members, should not be over friendly to
  men as is done in western countries. Kissing on the cheek, touching on
  shoulder or any other part, even shaking hands are not common between
  men and women. Store owners will shake hands with foreign ladies often
  but don't allow it to go beyond that with anyone. It is a sign of
  respect for yourself if you learn to separate Egypt from the west in
  how you deal with people.

I looked that up because I've just came back from a trip to the Middle East (Oman and Kuwait) and I was told by friends there that Public Displays of Affection (PDA) are illegal and if the authorities were to witness any PDA those people would be in serious trouble with the distinct possibility of being arrested.
Accidental kissing in that region of the world would be a whole different thing to it happening here in the UK (or The West). But I can imagine it still happening.
Here's how:
Two young people: stone-cold-sober (naturally) but under the influence of a potent hormonal cocktail - the kind of mix that makes rash-decisions the norm rather than the exception. They find themselves in the kind of freak situation where they are alone (trapped in a lift, left alone in a car in the dark for a brief moment while Auntie goes into the shop, classroom empties and they are the last ones out - let your imagination run wild).
They both know how they feel: horny but repressed. One starts to say something whilst moving forward half a pace; the other, with eyes as wide as a startled bird steps back. One of them trips; the other catches her fall; they touch; a spark flashes between them and before they know it, their lips have met passionately.
They both know that it's wrong but it feels so right!
Then a sound behind them and they spin away from each other - hearts beating wildly. Terror in her eyes and a mute appeal flashes towards him asking for his silence. A tacit agreement from his face; perhaps a brief nod.
The door opens and Auntie appears with a shopping bag in her hand. She looks from face to face and we know that she knows. She just knows. But she too was young. Once. She too knows of these forbidden things.
Everyone is quiet on the drive home - each caught in their own thoughts, feelings and memories. Both of the young people vow to themselves that this will never happen again. They make silent promises. Fervent oaths.
And yet still ...

