1. The grass is not, in fact, greener: there is no number of children an adult can have that will make other adults leave him/her alone about it. You go straight from, "You should have more kids!" to, "Sheesh, don't you know what causes it?" without any pause in between. Individual people may think they know an ideal number of kids for another person to have, but there is not a social consensus, so you just will not be able to universally escape busybodies on this subject.
I don't know if the childless or the people with ten kids have it "worse" as far as social pressure goes. Nobody appointed me judge and jury there. (Nobody appointed you judge and jury there, either, so please do not weigh in. Pain is not a contest, nor is annoyance.) I suspect that people who are unsatisfied with the number of children they have probably have it worse than people who are satisfied with the number of children they have, but as people vary, that doesn't mean I can say, "On the average, people with 2.875 children have it the worst."
2. Different personalities err in different directions. Some people are overplanners. Some people are underplanners. The existence of one does not negate the existence of the other. You cannot say that "the problem is," because we humans are so talented at coming up with problems that there is no one single problem. There are lots.
3. Not everyone has to behave the same way. Sure, humanity would have a tough time if each of the women currently in childbearing years had zero children. Probably would also have a pretty tough time of it if each of the women currently in childbearing years had ten children. That doesn't mean that individual women aren't reasonable in choosing to have zero or ten children or some number between. Personalities vary. People vary. We don't have to set out a policy that is universal on this subject. markgritter's parents were happy to raise four loved, wanted children. My parents were happy to raise one loved, wanted child. Forcing them to switch families or to average out to two and a half each would not have resulted in happiness for everybody -- possibly not for anybody.
4. Saying that every child should be loved and wanted does not dictate specific policy for what someone should do if they find themselves pregnant and do not want a child or do not want a child at that moment. It does not specify that adoption, abortion, some kind of parental support and/or mentoring program, living with extended family, or any other specific thing is the solution, either universally or specifically. It also does not tell the listener what the speaker would do or, indeed, has done in that situation.
5. Not everybody has the same resources. Telling people that they can do something does not make them able to do it. Be very wary of sentences that start out, "You can." Whether those are, "You can have kids later," or, "You can leave the kids with your mom," or, "You can go back to college later," do not presume on what other people can do. Sometimes they really, literally can't, no matter how reasonable it sounds to you.