That's a good point, actually.
However, since in the vast majority of instances the default choice is what's wanted (if I'm adding a child to a person with a spouse, in most cases that spouse *is* the other parent) efficiency would be improved by rolling the Select Second Parent dialog into the Add Person screen, something like:
Adding child of [Primary Person] and
( ) [None]
To go with the default, do nothing. But you still have the option to select something else if the occasion requires it. Fewer clicks, less frustration, greater efficiency, better workflow.