Sex feels good, right? If you haven't experienced it firsthand, I'd wager that your expectation is that it will (or would, if you're abstinent) feel good. Think about the natural things that occur to people: eating, sleeping, pain, starvation, etc. Apart from drugs, things that feel good are often good for the body. Things that are bad for the body feel bad. Eating and sleeping keep us alive. Starvation, pain (through heat, bludgeoning, stabbing, etc) are bad to us and are likely to end our life. The reason that these things have their sensation is because it is our body's way of programming our conscious behavior. It's reinforcing survival, and punishing what is harmful. Fear keeps us away from unknown dangers. Pride reinforces our success. Shame punishes our failures. Our body is telling us what will kill us, and what will keep us alive. Therefore, our body wants us to have sex. And since our body doesn't know about the idea of recreational sex, it wants us to reproduce.
Beyond the general good feeling, we feel the need to reproduce. We have this base impulse to create more of ourselves. In women, it can be referred to as being baby crazy, or the maternal clock ticking. However, many people would argue that we human are doing nothing good for this world or this universe. Seriously, how are we making the world better for the organisms around us? It would seem that our world doesn't need any more of us, that it would be better off without us.
So why does sex feel good? Why do we want to reproduce? What good will reproducing make? Why does this world need 1 more person? Honestly, I'd say the world doesn't need another person. This world doesn't need more people. We don't arrive at some logical conclusion that we need to have kids. We simply want to have kids. The reason that we reproduce is because we can, and our genes program us with the desire to do so. If we weren't programmed to have the desire to procreate, we wouldn't procreate, and we would die out. Any species that didn't have that desire died out, and what lives are the ones that did have that desire.
So should we repress this impulse? Should we do the world a favor and stop the spread of the human race?
No. If this planet dies, if it can't support our life anymore, what's the loss? If the Earth can't support humans anymore, we will die, and therefore suffer the weight of our actions. Without humans, the Earth will slowly fix itself, burying our poisons within the crust, sinking to the core to be broken down. Some plants and animals will survive what we have done, and those plants and animals will slowly clean up the mess, making a new world.
Of course, not all humans will die. We are nothing if not plucky and resilient. Our numbers will dwindle, to a level that Earth can handle. We will reform our ways. We will help restore the Earth to its former glory. We will either learn from our mistake and keep the earth clean, or the process will repeat itself.
So reproduce, but remember to do the Earth good. Clean up your act. Teach your children the proper way to care for our home, our planet. It's the only Earth that we will have, terraforming and traveling to another planet isn't going to happen. They're too far out, and there aren't many (if any) places that can support us.
We don't need to take care of the Earth for the Earth's sake. We need to take care of the Earth for our children's sake.