Three adults, possibly four, possibly more, all having little control over one another, have to negotiate a stressful arrangement optimally over a period of many years in order to not damage your children. It almost always doesn't work. I would call this borrowing trouble.
The same is true of having children and then separating in the first place, or ever going on to date. You have no control in those scenarios either. It doesn't start and end with having more children. Realistically the decision made that "damaged the children" was when they decided to leave in the first place, which nobody is condemning.
Should all parents who separate stay celibate and alone for the rest of their lives? Some might say so, to punish them for the decision to leave, but personally I don't see why you should give up everything else you wanted in your life going forward, in the hope that it creates the illusion that yes you might have left but it's ok and not damaging because your child is still the single and only thing of importance to you in your life. Unless you stay under the same roof as the other parent, the damaging thing has already been done and you will never live with that child full time. Whether or not you go on to find love or have more children won't change that.
Kidding yourself that this decision is only selfish if you go on to have more children is naive. The truth is anyone that has had a child and separated has already put their needs above their child's, and rightly so, because it's not up to a child to dictate who an adult should be in a romantic relationship with, just as it's not up to a child to decide if said adult goes on to have more children providing they continue to care for them as well.
Everyone here that has separated when they have children has already put themselves first, and have justified it to themselves, yet are unable to see that choosing to have more children is no more selfish than that.