Did you ex have a choice zippi? what if he had not wanted to divorce?
I think some people "see" this situation and others don't.
My feelings are that potentially, it will cause hurt- most likely to the parties involved as they will feel emotionally about each other, but know it cannot lead any where.
However, through out history people have had "affairs"- sometimes with their partner's knowledge and even consent ( think of Jane Clark- her husband the late Alan Clark even wrote a diary of all of his and she still stood by him), and other people have had discreet affairs that have actually saved their marriages by giving them an outlet ( and I don't just mean sexual).
In an ideal world, no-one would get divorced, or have an affair. But we don't live in an ideal world.
It is easy to condemn, just because you couldn't or wouldn't do it, but none of us is whiter than white - and without knowing all the details, we are not really in a position to judge.
Some people might divorce and others would condemn them, saying they should have put the kids first, or that their marriage was not really that bad....
interestingly, I think maybe that affairs were more accepted in my parents generation when divorce was just not possible really to a lot of women, and certainly frowned on a lot more than it is today.