There is an ethical dilemna inherent in having children anyway. Actually, come to think of it, there are several.
1) You will always want, but never be able, to provide the "best possible" environment for a child. In fact, it's possible that humans are so complex that it is impossible for a human to know what is the best possible environment for a child.
2) The kid didn't ask to be here. Sure, most 5-year-olds are busily enjoying life most of time (except when they are throwing tantrums because you won't buy them Chocolate Frosted Sugar Bombs)...but what about the ones with horrid degenerative diseases, malformities, dwarfism, etc? If you are the child's parent, it's YOUR FAULT that the child is this way.
3) What if you child, rather than finding the cure for cancer or being the next Einstein, turns out to be the next Hitler?
4) More people - especially in the Western, "civilized" world - means a greater environmental impact, which may reduce the time we have to find a way off this planet before being poisoned by our own wastes.
Of course, I've neatly sidestepped all of these by deciding that I'm not going to have children for reasons that have nothing to do with the above dilemnas.