I would entirely agree with this.
Personally, if I know from the start that an ex has gone off with someone else, then they dont even get the curtesy of it hurting me.....call it whatever you will, but that to me is the end all of a relationship. final! never go back because they will do it again, if not to you, to the person they went to (I've seen it done many times)....personally very easy to move on, the ex simply deserves no respect.
I have no idea if this is something that is ingrained into me from childhood, but I expect to be treated as I treat others, I certainly would not do that to anyone that I ever had an ounce of respect for, even if my feelings for them changed and do not expect it done to me....had it done though! and those ex's were happy times to let go of.
However, if an ex has let go, then months down the line got together with someone, then I suppose its down to the individual?...there is no magical cure for avoiding it hurting.
Its almost something that you need to expect, a few weeks ago this thought would have killed me, but now, I am happy with it, as long as shes happy, as I know its bound to happen at some point, I would never hold it against her if she ever wanted to come back (and indeed it may be one of the reasons that she wants to come back after seeing that being with someone else is not as good) but I doubt now she will ever come back from a realistic point of view.
Its all just down to the great cliche of "time"..none of us know how long it will take for things to get easier....sometimes things can just hit you overnight and you will be fine the next day from that day onwards, sometimes knowing they have met someone else can help speed healing up and others is just a long slow healing process, I've been through all, but usually something just clicks one day or night that makes me re-focus suddenly.