I only have one child. Perhaps I am lucky that I have no experience of people giving such offensive comment, or perhaps my friends are all sensible people. Of course there will always be ocasion that people simply ask do you have intention of one more. Which I will just say no and move on.
If anyone dare to cross that line and say anything more, I would tell them to mind their own business. After all, this is a personal choice.
I don't think having just one child means selfish. Instead, I would say if having more than one child with no intention/ability to give them attention, support and love that they need, that is selfish and irresponsible.
This generation, there is a tendency of smaller family. Definitely one reason is a lot of career women started their family later, and also most people are sensible and consider whether they can cope financially, physically and emotionally.
I am not a single child, but life turns out all my siblings live in different part of the world. Besides, I do notice that through out childhood, we all have our own friends.
Sometimes I am surprise with the endless social engagements my little one has. That literary filled up the family calendar. Will be a joke if anyone ask whether she is lonely.