Hmmm. Interesting topic. I think its pretty easy too take the side of "selfish" when it comes to not having kids, but its more of a pointing out a speck in your neighbor's eye, while you have a log in yours.
On the one hand, if you want to have kids, and have the means to take care of them, go for it. Obviously if we dont have children as a species, we would die out. Duh. But also, its the only way to lengthen your families linage, bloodline, and name. A lot of people, male and female, cant wait to have offspring. Its relatively easy to do, at least to get the ball rolling (so to speak). There are tons of surprise babies every year.
On the other hand. There are a multitude of reasons to not have children as well. You might possibility be biologically unable or you partner might be. You (or your partner might have a trait, disease or something that you dont want to pass on to a child. In these cases, there is always adoption as an option for obtaining (lol) a child.
Now that those thoughts are out of the way, on to the more personal decision forks. Personally, I am terrified of having children. My job only pays so well, and I know raising a child costs quite a hefty sum of money. Buying supplies, toys, clothes, and medication, and everything else I cant even begin to think of. And thats only the first few years. Later down the road, there is school, possibly a car, helping with college... The amount of money you pour into this child is staggering, let along have 2 or 3. Now, the buffer with this, is that love that a parent pours all over their relationship with this child. Bringing them into this world, and watching them grow, thinking "I made this". It changes you. I know my was more on the side no children fence. But my mom wanted at least one. I was the one that changed my dad forever. He said there is nothing on this earth as powerful as your own child. You have things you might love. You have your own parents you love. You love you husband/wife more than you love anyone else. But a child. Your child. When you see you child for the first time (according to my dad) you cant believe the amount of compassion and love that wells within your being. You know in that instance that you would do anything for this child, as long as it lives. Now that is the typical response, I think. I know there are parents that dont love their children, or dont express it in normal ways. That is just my experience.
Other reasons I have heard or possibly more selfish, maybe. One is that having a child wrecks the woman's body for a good while, or for good. (C section, stretch marks, irreversible weight gain). And that can be hard to deal with. Never being that same pre birth mother again. Selfish? Maybe? Just because you want to keep you body? I dont know. Thats a personal choice. Another, probably a smaller percentage of individuals, might have come from large families. One older fellow I work with is married to a woman who came from a family of 10 bothers and sisters. She was one of the older ones, who spent most of her early years taking care of all the children that came after. Even though Billy ( my coworker ) wanted kids, he respects the opinion of his wife and settles for a few dogs instead. I know its not the same, but they both have had quite the adventuresome lives despite no children.
There's a third side of this coin (the observed super position if you will) where there are couples that have kids that certainly shouldnt, at all. Unfit parents who have children, raising them in broken, abusive homes. Something else to consider, maybe would be parents with no kids is actually an unseen blessing.
Thats most of what I have now. I think its situational whether its selfish or not to not have kids. At the end of the day, is it really anyone's business than your own?