Interesting thread. I agree we shouldn't demonise people for feeling tempation, or even acting on it. They are not bad people, but make no mistake, deceiving one's partner is a bad thing to do. It's horrible, selfish, and even abusive behaviour, especially if the "extra" relationship causes the deceiver to treat his or her partner badly all the while it is going on.
It is however because I understand tempation and recognise human fallibility that I do think it is equally bad behaviour to send signals to someone who is attached. Couples go through all sorts of ups and downs, especially in a long relationship. Even if the primary relationship is fine, people go through all sorts of psychological ups and downs.
When a couple are in a "down phase" - or a person is at a low point, tempation is much more difficult to resist. The buzz of someone new, the flattery and the attention etc. can be much more intoxicating at this point. Someone who capitalises on this situation is not being a "friend" to the person at all - offering them an affair is actually more likely to cause the person untold mental anguish, not to mention that caused to a whole load of other people who have no choices (partners and children).
I can think of several men who over the years have come to me as a shoulder to cry on, because they have been having problems in their marriages, or were feeling depressed without knowing why. I'm afraid I've also known that some of these men would have also been very tempted to have an affair - but to do so would have been disastrous for them.
Instead of offering an illicit relationship therefore, I have suggested counselling - or got them to see the situation from their partner's point of view. Got them to evaluate their relationship and talk to their partners about how they were feeling.
It's how I would want someone to behave if my own DH was feeling depressed or even dissatisfied with our relationship.
Everyone has the choice to say "no" and in an ideal world, every attached partner would have the strength of character to realise that an illicit relationship is not the answer to what ever problem they are having, either within themself or in their marriage.
But people have weak points and weak times and it is part of the human condition to feel temptation. So the decent thing (and I believe the way we would all like people to behave towards us) is to recognise that - and not capitalise on it. People who do so are in effect delivering a fatal blow to someone's relationship, when perhaps all that's needed is a bit of tending, or some treatment if one of the parties is depressed.
I agree also that people tend to overlap in their relationships, when actually the more grown-up and sensible thing to do would be to have some time out on their own. I'm also sure that for some people, especially those in abusive relationships, it is only when someone decent offers an alternative that people get the strength to leave, but even for those individuals, jumping into a new relationship might not be the best option.
I'm afraid I've met many more people however who have convinced themselves that the "new" person is much better, leading them to re-write history that the "old" partner was bad for them etc. Some time down the line, after surveying the wreckage of hurt children and a fractured family, they have concluded that the new person wasn't the panacea after all - the new relationship actually isn't any better than the first, the partner left behind was actually better for them and it was all a dreadful mistake.
OP, in your shoes, I wouldn't want to be a rebound love interest. I would prefer to be with a man who ended relationships because the relationship was wrong - and not because a better alternative came along.