I think the answer to this lies in the potential for postive to potential for negative impact ratio. I think all 3 parties could suffer a negative impact at some point in an affair - the spouse having the affair through the guilt and upheaval they cause themselves, the spouse not having the affair - should they find out it is pretty damaging to have such betrayal - and the single party having the affair due to the potential upheaval, insecurity, loss of control in the relationship, guilt at causing pain to others. Add to that if there are kids involved, if there is an ensuing messy divorce, the general emotional messy aftermath of an affair, the unconscious sense of being an immoral person there's a fair amount of negativity potential on the cards! Statistically affairs are down the list of successful starts to relationships due to the emotional friction involved between all parties. Positives are moving to a more successful relationship, avoiding years of relationship stagnancy and an end to an unfulfilling relationship. I don't buy that there's any long term gain to the excitement of an affair, the secretiveness etc..

On the whole it seems to me that an affair is largely a reaction to problems in an exisitng relationship and a path of least resistance to a perceived better emotional life. I don't however think it's the best way to proceed for anyone unhappy in their relationship. Best to address problems you have now. There's a good chance if relationship problems exist in your current relationship then you are partly responsible for them. If you buy a new car every time it gets a flat then you're in for expensive disappointment!

I guess my main feeling on the question is affairs hurt people - and not just the person being betrayed. Bad idea.

I don't think it is OK to have an affair. If you are in a relationship and need to have an affair, the relationship is not good for you. Why not get out of the troubled situation and go with another person?
Cheating is not good for your physical and mental well being. It will create more stress in life.
If your partner deserve to be cheated, what is the point in having a partner? What about your own values?
In a relationship people are committing to each other. If one is having an affair, the commitment is broken. It is not fair for the other person.
Also, if safe sex methods are not taken while having the affair, there are risks of sexually transmitted diseases. You are exposing the unsuspecting partner in the relationship to these diseases also. If there are kids in the relationship, what you can tell them?
The kids will be happy and healthy in a family where trust and love is important. It will be better for everyone involved, if you get out of the relationship and pursue your happiness.
I don't believe there is any justification for having an affair, unless you are the owner of Ashleymadison website.

No, I do not think it is okay to have an affair. I was the victim for years. My ex-husband had many and I could only beg him to go for counselling. Unfortently he refused and the marriage ended. It left big scares on me in many ways and I still wonder to this day why he felt it was okay. He is now remarried and I know he has cheated on her as well.
http://www.flickr.com/photos/amsterdamned/2037617367/

It's pretty crappy for the other person if you have an affair behind their back. But if you are not in love with that person and you fall in love with another and you're too much of a coward to end one relationship before you start the other, then I guess cowardice could be a justification. If you truly love your spouse though, then having an affair just for the physical pleasure is not justified. Tell your spouse you are not satisfied with the physical part of your relationship to at least give them the chance to discuss options before just taking matters into your own hands. Unless of course you don't believe in the sanctity of marriage in the first place, then by all means do whatever you want and hurt whomever you wish. Who are we to judge anyway.

If you are in a relationship with someone and feel the need to have an affair, perhaps you should re-examine your relationship and the reason you are considering or having an affair.
I don't think that there is any circumstance where an affair is justified or acceptable. If you are having an affair and do not feel any guilt or do not realize that what you are doing is wrong and hurtful, then there is something really wrong with your relationship with your spouse or partner. You should either work on the relationship with your spouse or partner, or get out of that relationship.
A commitment to a partner is just that...a commitment to which there are no grey areas.