Yes! Having an only child is selfish!
Having children is selfish.
DH and I chose to have 4 children for selfish reasons.
I was lonely growing up, and used to look at big families wistfully, wishing that my family was like that. I was trying to prevent any child in a family of mine having those feelings, IYSWIM, because although I had a sibling, I still felt lonely. DH grew up in a big family, so it was his norm.
It was still based on selfish reasons because it was what we both wanted.
If you choose to have one child, you are preventing your own child from a sibling. I'm sorry, but IMO this is selfish, unless having another child would cripple you financially, or massively negatively affect any physical or mental health issues within the family.
If it's just a case of "DH and I get to relax and have a lot more time to ourselves, we have more money, and can have better holidays," etc, then that is selfish, surely?
My sibling and I never got on until the eldest of us turned 18. She was horrible to me when I was growing up. I didn't wish that I was an only child, I wished that I had additional siblings who would be nice to me, because I could see from other families that most siblings are nice to each other most of the time.
She is now a massive part of my life, and so are her children, and I can't imagine life without them. I would hate to be an only child, and given the choice as a child, even though she was so nasty to me most of the time, I wouldn't have chosen to be an only child.
It's crops up time and time again in the one child family threads that siblings don't always get on any way, and often hate each other and go NC as adults.
I know hundreds of people with siblings, but know of only one case of adult siblings hating each other and being NC.
I'm speculating, but I imagine the divorce rates are much higher than the number of adult children hating each other, yet it doesn't stop people getting married.
IME, of speaking to one child families, who are in stable relationships, with no medical / MH / serious financial worries, the parents either just found it so hard first time round and/or hated the early months so much that they didn't want to repeat the experience.
Either way, having one or more than one child by choice is selfish!