I suppose it's the difference between asking "is it better to blow up the train" and asking "can it be better to blow up the train?" It's worth noting that even if we have an obligation to create lives, our obligation to save them is easier to fulfill; but it's still worth knowing if the two are actually equivalent.

Reading the original comment, adam does, in fact, seem to have assumed that having children would be the right choice if it mattered, so ... point, I guess.