I genuinely can't even imagine a scenario where someone would make a negative comment to me on having one kid. I was and still am occasionally asked if I wanted more, and the cheery. ."God no, only ever wanted one" always put an end to the topic. Probably that's why I'm bemused.
My daughter says she has occasionally been asked if she wanted siblings and her cheery "no way" puts an end to it as well, as said, most of her friends are jealous as they often can't stand their siblings, and most folks I know often wish they'd only had one as much as they love their kids.
I have a brother, i can't say he was my play mate or anything like that, I had my own friends. In fact he was a pain in the backside. People I know who have more than one kid, their kids are the same, they have their own individual groups of friends and often don't even get on that well.
I never busted a gut to organise stuff, or tried to get my daughter to play on her own, if she wanted to be with friends, she told me, we took her and collected or they came to the house, if she wanted to be with one or both of us, she was, and if she fancied doing something on her own she did. It was simply natural and organic.
My husband did more of the "playing" than me, as in football in the garden. Going to the park etc, playing monopoly or whatever, as I have a low boredom threshold and he loved all that, I was more the care giver, the talker, the organiser.
Now she's 19 and at uni, he stills is the more playful one, i.e. He plays PlayStation with her when she comes home and I sit and chat to her and watch crap things on tv which make us both laugh or buy her fav things in to eat.
It's about being loved and included.