Me and husband have made the decision to only have one child (mainly because of health problems I have) and when I told this to my mom the other day, not the reason behind it, but just that we only planned on having the one, her response was one of shock/horror! She passed comment to imply that it wasn't 'right' to just have one as it wouldn't be fair on the child.
I have a sister who is only 12 months older than me and I love her to pieces. We are best friends and we had the most wonderful childhood together and part of me feels like I'm doing a disservice by not providing a sibling. My husband has a brother who he can't stand, they never got on as children and still don't as adults, they don't have anything to do with each other. His brother and his wife/children didn't even come to our wedding. It goes without saying my husband isn't too concerned about the supposed 'benefits' of having siblings.
Is it selfish to just have one? 
I didn't think it was but I can't get my mom's response out of my mind...
I did explain our reasons to her and although she took them on board she certainly wasn't too convinced by the argument.