To me it makes no sense to deny that there is a human overpopulation problem. Most of the people who say overpopulation is a myth are those who already have a few kids. They throw up stupid arguments like "Everyone on the planet could fit in the state of Alaska and we would all have 2,000 square feet of room each", but fail to explain how each person would produce enough food (clothing, etc) for themselves in that 2,000 foot space, where their waste would go, and if they would be willing to share that space with their kids, grandkids, and so forth.
The over population deniers spout crap like "The earth provides enough for everyone's need, but it is just not being distributed fairly". Which is only partially true and it negates the fact that most people are over consuming... if everyone lived like the average American, we would need 4 or 5 earth's to sustain us all! Perhaps the earth would provide enough for everyone's need, if everyone in the developed nations did not consume as much as they do now (not just food, but other resources too). However, when I see somebody saying that the world is not overpopulated they represent the typical first world person; living in a roomy house, driving a newer car, owning electronics and things that are otherwise not true "needs". They have no idea what a mess the world would be if everyone had an equal amount of stuff as they had.
Deniers don't see the obvious things as signs of overpopulation. I refer to farmer's fields and other avenues of agriculture. Do they think that farms have always been where farms are now? Does it not occur to them that in some cases that land used to be a rich forest? As soon as the very first forest was cleared and that land turned into a farm, we impacted the earth negatively.
As soon as we drove another species to extinction, either because of habitat loss, or because we over-consumed it, we were a problem. As soon as our waste started to pile up we should have clued in that we were a problem. Why can people not get that?
Are they so wrapped up in their own reality that they fail to see the global picture, or are they just in denial because they have to be in order to justify pumping out kid after kid?
I rarely bother to argue with ignorant deniers any more. I have conceded that in most cases they will not be swayed, especially if they already had kids and have to maintain the fact that they are not contributing to a potential major concern. Instead I will focus on educating the youth, with subtle comments here and there, in hopes that they have open minds and will absorb some of what they are told and will go forth to expand on what I tell them.