People seem to be focusing on finite resources and overcrowding of the planet though. And about the risks to our economy and our society posed by having a declining birth rate and a burgeoning elderly population.
There are concerns about those things of course, particularly as many of the families who have very high birth rates are also often the families who statistically tend to have higher levels of social, economic and educational disadvantage and therefore for the next couple of generations at least, will probably need to take more out of the pot than they are able to put in. I do wonder if economically the welfare state is sustainable. More and more people are choosing to have just one child with maybe a second as an indulgence if they think they can afford it because they are having to work harder and longer for less reward. Their children are already in full time childcare just so they can both work to meet the rent/mortgage. A large family simply isn't an option most responsible, financially self sufficient people can dream of these days. They are having to get real and cut their cloth accordingly.
So it does frankly really piss them off to see others having six, seven or more children they can't support or house without quite a bit of financial input from the state and expecting the taxpayer, i.e., the very people I've just talked about, to take up the slack for them so that their children don't need to live in poverty or disadvantage. And adding insult to injury by being all whiny and entitled when the HA doesn't have a bigger house available or when benefits are capped. You want six or eight kids then they are your responsibility not mine or anyone else's and if you are struggling and they are going without then that's a direct result of your decisions and lack of personal responsibility, not mine or anyone else's.
The irony is that the way our benefits system is set up, you could be an unemployed parent/s who has chosen to keep adding to your family knowing damn well you are either single and/or unemployed or working part time on NMW and your eight children are probably not going without any more than a full time working couple on an average wage with two children are, because when you are dependent on benefits to a large degree, the children themselves become cash cows, your route to a bigger better house and a means to an end.
But for me the selfishness and irresponsibility of adults choosing to have too many children ( by which I mean six, seven or more, I don't consider four or five to be particularly excessive so long as you can afford them) is more about what goes on in the family home itself.
I know it can be a hard and slog and there are frequently not enough hours in the day bringing up just three children, in a spacious house with a comfortable income. The thought of having double or even triple the number of kids, with less space and less money and less time for them horrifies me. No-one can do a job really well when they spread themselves too thin - why do we think parenting should be any different? Okay, some people are super organized, superhuman forces of nature but most of us are not. There are limits to our capability and endurance and when we take on too much someone suffers.
As a SAHM it seemed there was never a day when someone didn't need something from me that was very important at that moment, whether it was the dentists, getting dog poo of their shoes, making meals, finding time to BF, taking to ECAs and picking up again,, watching an assembly at school, parent teacher consultations, nursing them when they were poorly, settling them when they woke n the night, helping with homework, doing nightly reading practice, reading them a bedtime story, supervising bath time, taking them to a birthday party, hosting a play date, or rushing to turn around a sports kit in super quick time because there is a match after school tomorrow and it got covered in mud today. The match of course I would then be expected to stand and watch, with my other bored kids in tow. Then you have to deal with the tantrums, whether from a two year old or a hormonal shirty sixteen year old. Even with only three kids and no job (SAHM with working DH) there were times where I was on my knees with exhaustion (and a couple of bouts of PND) and just wanted to lock myself in a room and be alone, but you can't.
Or just having the time to sit and hold one of them and listen to them and only them and give them all of me for just half an hour now and then, without some other pressing priority or interruption from a sibling. That's what I meant by 1 on 1 time, it doesn't necessarily mean offloading the others in order to take each child out for the day alone. I'm not sure I ever felt the need to do that.
Then there's the issue of space and money. My children never had to share bedrooms. It's not the end of the world if they do, especially when they are fairly young but having to cram three or more children of mixed ages or sexes into a room where there is barely enough space for sufficient beds and no space for them to make their territory feel personal with other belongings of their choosing seems a bit of a crappy way to choose for your kids to have to live if you ask me.
And teenagers hate their space and privacy constantly invaded by younger siblings, it's stressful for them. Where do you go to study quietly? To take your friends and talk about stuff without a bunch of small kids earwigging or competing for attention? How do you watch what you want on TV without the little ones seeing things they probBly shouldn't? Every psychologist knows that overcrowding, constant noise from others and a lack of privacy and personal space can drive people demented with stress. That's what prisons are like.
Ask any teacher about the dynamics of a class and who inevitably gets the most attention and the least. Some children have much more assertive or needy personalities than others and they demand more attention. Some are troublemakers and bullies who need a watchful eye at all times. Some have issues or disabilities that require more of your attention/time. Some are the self sufficient copers and carers of this world; the ones you task with the job of looking after one of the needy children at break time while you have your coffee in peace. Sometimes they might feel insecure and in need of attention too but they don't show it because they've been conditioned not to and besides, they see you are busy tending to the empty vessels and the trouble makers and the ones with difficulties. Others are the underdogs who get pushed out, ignored or bullied. The bigger the class size and the fewer support staff you have, the harder it is to keep on top of who really needs your attention, as opposed to who is actually getting it.
I think the same is true of families. You simply cannot do for eight or ten what you can do for three. You can't.
Sleepovers and playdates get dismissed as an annoyance and an inconvenience. Birthday Parties go unattended and unhosted. Sporting prowess and talents get overlooked and unnurtured because no one has the time to run around to clubs and matches, or worse still, you expect other people with a regular amount of children to do the running around for you, because they have more time and more space in their car. 
There is always, even on a high income, less money and space and food to go around (unless you are single and on benefits, in which case there is more.)
And don't get me started on how I feel about bigger children (usually girls) WHO ARE STILL CHILDREN having to speed up or suspend their own childhoods in order to take on caring responsibilities for too many younger siblings because mum and dad can't cope day to day with what they've saddled themselves with.
Also, even if you dismiss all of the above, insist you are fine and cope marvellously, no one is disadvantaged by your choices, then your next child is born with profound disabilities or health problems that require enormous amounts of your time and enormous changes to the way the household is set up and the routine is run?
Or you suddenly become seriously and chronically unwell or disabled yourself and unable to cope with the day to day needs of your family? Or God forbid, you died? Those would be a tough things for children to cope with in a family of any size, but undoubtedly the more children you have, the more of a practical issue it becomes, particularly if you are a single parent with multiple fathers to your children. There's a fair chance your children would have to be split up and fostered fostered separately, or that the eldest child would have to take on the role as parent when they might still be barely adult themselves.
Alll anyone should want for their children is to be able to give them the best of ourselves. For DH and me that means not spreading out time, patience and resources too thinly just to scratch some continued biological itch, or because some religious leader told us to, or because it's cute to always have a baby around.
The children you already have deserve better than that.