Funny - I came to look at this thread ready to pitch in at anyone who said 'yes, it is selfish', but not at the OP. I don't read that as offensive at all.
Yes, lots of people come up with stereotypes about only children. They're not even in that category of having become stereotypes because they're true. They're bollocks. DD is quite shy and quiet, which some think are only child traits. Thing is, I'm quite shy and quiet (eldest of two) and so is DH, sometimes (youngest of two). We all like our own way. Mind you, I've not yet met a 5yo who DOESN'T like their own way ... or many 37yos for that matter.
My mum is an only child. She had quite a bad experience of it, I think. I suspect she only had my brother because she really didn't want me to be an only ... but she was actually pretty much brought up with her cousin, not a 'real' only at all.
It's rubbish distinguishing between one-through-choice and one-through-circumstances. It's still one child at the end of the day. You can't claim a child will suffer if their horrid parents didn't want another, but not if their poor martyred parents couldn't have another. It's the same thing at the end of the day - although damage could be done by beating yourself up about your choice, or not being able to get over the grief of your circumstances. Whichever way, you just need to be at peace with what life has dealt you, and love the child, or children, you have.