It looks like debate is over on this thread, but I want to add my comments for people who find this in future.
I have also been thinking about this question, as I have 2 children and would love to have another but feel this may be rather selfish. This isn't about judging other people - it's about me thinking through the decision I am about to make. I can't look at or think about another person or baby who actually exists and think, they shouldn't exist. I am only thinking about the choice I myself can make now.
I think this is a very important question to be able to ask ourselves and talk about and I am glad that newwave has raised it. Me thinking about this for myself is completely separate and nothing to do with questions about government policy, sterilisations etc. I am thinking about my own personal and voluntary decision that I am going to make, on the basis of my own thoughts and feelings and belielfs. I find it is helpful to be able to share these with other people and learn what they think - that is why this post is so vaulable - I imagine that is what several of the mumsnetters above think (it has nothing to do with being jugemental).
I would also say, I think it is not only about the exact number of children one has, but about any decision to have another child. 'Two children' probably comes up because few people would deny anyone the right to have a child if they want one, and many people prefer their children to have siblings; and two is (almost, allowing for mortality) replacement for the two parents. But anyone considering having a child could consider the issues involved in this question, regardless of the number.
To me, 'the issues' are the fact that a person living my lifestyle - me! - consumes a huge amount of resources and this has a big impact on the other people alive on this planet - today, and in the future. Even though I care deeply about justice - environmental and social - I know that I and my family have a big environmental footprint (much larger than the average same-sized or even larger family) living in a developing country. I also know that the actual actions I take to minimise these impacts - and make positive impacts, like lobbying for change on issues I care about - are not nearly as significant as they should be or could be. We have one car for the family; we walk or cycle or catch the train when we can, but we also use the car several times a week as we live in a rural area; we grow some of our own veg (also badly); we spent extra on rennovating our 1960s house to include extra insulation, triple glazing and a more effcient boiler - but there is some draught-proofing that could be improved; we try not to fly and to holdiay in the UK except for important family occasions e.g. weddings, rare visits. In other words, we do a bit to act on our beliefs, but are far from perfect and still consuming more than our share of the world's resources.
My children will probably go on to be much like me; certainly they will go on to have their own homes and travel and consume in their own right - even if they try to do this in as ethical a way as they find possible. And they are likely to have their own children. My siblings have their own children. We are all consuming a great deal of resources, and creating a good deal of pollution and this all has impacts on people whose villages and towns we drive through, on communities who live near landfill sites, on people living in low-lying coastal areas lost to sea-level rise, and on our selves and our children as our climate and landscapes change and wildlife is lost (more housing, roads, car parking, sewage, water extraction etc) and we witness (and perhaps even experience in this country) more conflict related to access to resources such as water and oil - and peace and quiet.
This all sounds very gloomy. I feel I must say, then, that I love my children (and my DH) and I love our life as a family. Our days are (mainly!) happy and joyful. I would love to have another child, because it is such a beautiful experience to nuture a baby, because it is a pleasure beyond words to see our DCs play together, because I have gained so much in my own life from having siblings, because our DCs are such characters I can't help feeling there is another one waiting to come and join them.
But I am worried (almost convinced) that if we do, it will have been a selfish decision, given what I know and believe. How sad.