The selfish thing gets thrown around a lot, but guess what? Everyone is selfish. Basically every choice is selfish. Ask any parent why they had kids and it will start with "I want/wanted." 

Besides which, having kids is certainly not the only way to contribute. We all pay taxes. And guess who is paying for your kids to go to public school, while never having kids of their own use them? Yup, us. 

Guess who is funding your playgrounds and parks? Yup, we're putting money in on that too. 

It isn't all about being so obsessed with getting something out of the world. There are plenty of us who look at the tax breaks parents are entitled to and get pretty pissed off when we don't get nearly as much back compared to what we pay in. We're often subsidizing YOUR choices---not the other way around. 

Not to mention the fact that there are MANY reason why someone might opt not to have kids. Better to not have kids than to be a shitty parent. No one made you the life police. Get over it.