I'm firmly in the its a bit selfish to only have one child camp
I'm firmly in the camp that its selfish to have child after child when you can't support them.
It is selfish to bring even more children into the world - we just can't support the 70-odd billion people we have right now.
It is selfish to assume that you have the 'right' to housing, good schools and financial support because you have chosen to have children (see point above).
It is selfish to have a child purely as a 'playmate' for another child.
It is pathetic to actually spout crap about 'cruelty' and how an only child is missing out, will be lonely, psychopathic etc. Or to ask crass questions about their sex lives to parents with large families (another deeply offensive thing).
I have met loads of only children (for a variety of reasons - and yes, I do ask them) and not one has said 'I wish...'.
For some odd reason I went through a phase of wanting a twin (and I have quite a crowd of siblings) but that didn't last long.
The most unbalanced/unhappy people I have met have been either spoiled rotten to a rediculous degree by their parents, or a child from a huge family who has been neglected because they are from a large family (not enough resources, time, energy etc to go around - or they have been used as a 'spare mum' to their youngest siblings). Or people who have had child after child on some religious compulsion.