Ok, let me start by saying that I've never been cheated on (as far as I know) neither have I ever cheated on the context of a monogamous boyfriend/girlfriend relationship. So I apologize in advance if my opinion feels thoughtless to those of you who might have been hurt by cheating, that's not my intent.Alright I'm sorry I've to indulge into this deep argument but I can't help myself to do healthy debate. I'll try to keep things short.
The purpose of cheating do not justify cheating at any cost. I know there is a thin line but cheating on your partner with someone who is going to be your next relationship than cheating on your partner with several people without any intention of breaking up your relationship IS ALWAYS SAME. END RESULT CHEATING! If you want to change partners, is it necessary to cheat? Why not discuss things with your current partner and breakup by CUMMUNITCATING instead of indulging in this act. I'm absolutely against any type of cheating (unless that is kink or fetish of someone but other one should be aware before indulging in this kinda act and keyword is Communication). Now these things fall into categories of cheating which you mentioned and which you didn't mentioned. Cheating is the most disgusting thing that you can do to your partner who put his/her/anything in-between faith, trust, belief, who values you, who is through your happiness/sadness and all other kinda times. There are temptation's and your partner is right besides you to overcome his/her/anything in-between temptations and help you too so that you both don't fall into compulsion. Desire and temptations are bearable and not bad as it is human nature BUT COMPULSION NOT. And cheating is a sort of compulsion.
I don't think cheating is justified, it's wrong. My point is that not all kinds of cheating are equally wrong. Human relationships are complex and sometimes the end of a relationship kind of blends into the start of another one. That's specially true for people afraid of being alone (not my case), they might find very difficult to end a relationship without the prospect of a new one. That prospect new relationship meaning there has already been sex with the new person or not, I consider it cheating anyway and don't think that the sex makes such a big difference (which was my point for the Ian/Cindy thing). Also the soon to be ex it's going to be hurt by the ending of the relationship and (probably) the start of the new one wether there was sex before the breakup (assuming they are aware, it they don't it's kind of a victimless crime) or not.
So yes, cheating is a bad thing, but just as many other bad things we somethimes do during our lifes, it doesn't mean that we are going to do them again in different circunstances. Just as someone never cheating on the past, doesn't mean they will never cheat in any possible future situation.