Annibc, first of all, no one is guaranteed that they will have any children, even with 2 perfect tubes. You may have some, you may not. The point you seem to be missing is that continued fertility in any way has NOTHING to do with the moral arguement. For the morality of the action has nothing to do with fertility being destroyed or saved. It has to do with the life of the child.
I will try to present some more details of why this is considered so. What we are talking about is Double-Effect. That is a theological position that states a good act can have a bad side effect, but that doesn’t make the act bad. The part that gets tricky is that people tend to reverse this so that a BAD act that has a good effect becomes good. That is against church teaching. Bad acts can never be done, even for good to come of them. The short version is that directly killing the child by removing it alone is inherently a bad act. There is nothing wrong with the child itself so removing it to leave it to die is a bad thing. If they could remove it and put it where it belongs, or in a incubator until is grows up, then that would be good.
So why is removing the tube considered good? Well the difference is first that the tube is not a person, and doesn’t have any sort of inherent rights. It is a part of the mother. In an ectopic pregnancy the child has usually gotten attached someplace in the tube. The tube was not designed for this, so it starts to bleed and break open. This is what threatens the mothers life. Here is where it gets a bit tricky. With current medical limitations, all ectopics will end in the death of the baby, but as mapleoak has pointed out, many would also resolve themselves naturally, with no real danger to the mother. However, it is also very possible that people are unaware of the ectopic until it has reached a time critical stage (rush to the hospital, the mother is in danger). Thus, many doctors will pre-emptively try to avoid the possibility, before it even exists, simply because they know the child has no chance anyway. This is what is wrong. Hastening or even directly casuing the child’s death in order to avoid the mere possibility of danger to the mother is wrong. In most cases where the tube has started bleeding and begun to rupture the child is still alive and growing, which is what is causing the rupture. Thus you are directly killing the child by removing it, even if you are doing it for a good reason, namely to save the mother. Bad act with good end does not equal good act.
So again, why is it ok then if you remove the tube? Well, removing a bleeding, broken tube is not in anyway a bad act; it is a good act. But by doing so you will certainly kill the child, which is attached to that tube. Everyone knows this, it isn’t like surprise! you happened to kill the baby too. But we do not have any option that will save the baby. If we did, we would be morally obligated to take it. Since we can not either sit by and just let the mother die, we have to do SOMETHING to treat her. And simply, there are no other options that anyone knows of that will save her, and you simply must stop the bleeding, so you either remove the ruptured part of the tube, or the enitre tube, depending on your choice and that of the physician. I don’t know of any other way to repair the damaged tube and not hurt the baby, because the baby is always right where the rupture is and so small that any treatment will kill it along with accomplishing the treatment. Thus treating the wounded tube is a medical therapy (good act), even though it has a bad effect (killing the baby), even though everyone knew that it would happen. Again, if there were ANY option where the life of the child and the mother could be saved, we would be morally obligated to take it, and then removing the tube and baby would become morally wrong.
Personnaly, I would opt for removing the entire tube rather then sewing it back together, but that is a personnal choice. Research has shown that having had an ectopic greatly increases chances of future ectopics. It is only logical; there was a small flaw in the tube that the baby caught on in the first place. By attaching and growing there, it made the flaw/scar tissue/irreguality even larger so that next time around the baby has an even greater chance of getting caught again. Thus you end up going through more painful episodes of having the surgery, or knowingly talking drugs to directly abort the child. That is too high a risk for me. But for others, they might choose to try to repair the tube, and there is nothign wrong with that.