Hi again,
By succumbing to temptation, I was referring to the temptation to
have another, whatever the reason may be. It could be love, sex,
attraction due to gestures of affection, boredom/monotony (need to
look for a new 'toy'), or money. Or a combination of the
above.
As you said, its unfair to both.....but it does happen. And often,
the one outside (the 3rd party) will know that he/she is being
two-timed too...but doesn't mind.
Nowadays, there are also cases where all three parties know, but
doesn't mind....especially in the case of swingers. This is a case
of two-timing purely due to sex. In anticipation of your possible
next comment that love and sex should be associated with each
other.....I will say that many people can separate love and
lust.
Rest assured that you're not the only one who is confused about
love and gestures of affection - I'm a guy and I'm just as
confused! 
Its hard to sit down behind my PC and say whether I'll forgive or
not. But I suspect that it will depend on each case.....and the
severity of it. It will depend on who and why. Maybe I will not
only forgive, but will improve myself? I know some cases where
people have two-timed, but later become very faithful (ie.
temporary distractions).
PS: I suspect that a barrage of replies may come in after this
post, with wannabe angels preaching about morality, etc. 
