Why is it that when you have only one child, you tend to get questions and advice from people you meet about having a second one? What's wrong with having only one child? "She will be lonely", "One child is not enough", "Don't worry that you can't afford it, cross the bridge when you come to it" are the add-on comments. I try not to rebut most times as it is just a waste of effort.
Though these people may mean well and the concern is appreciated, I think they could have forgotten to be more sensitive to others' feelings and respect their choice, (and mine for instance).
Do they know the whole story? What if the couple is actually in a dilemma about whether to have another child or not? What if they have actually been trying hard and not been successful? What if there are other circumstances beyond their control?
On the opposite scenario, I doubt if there are many only-child mums who go around telling people NOT to have more than one child. I for one don't ask "How are you going to cope with so many kids?", "Aren't you thinking of getting a hysterectomy?" etc...