It's selfish, and yet, it's not selfish.
There's literally so much focus going on in the just about 7 billion here that a lot of people just miss certain points entirely. Take this scenario(that I've been thinking of): what if every single family adopted every child that was up for adoption? Of course, that would place the kids who are in foster homes actual homes to live in, yeah? Excuse me for digging a bit deeper, but whats stopping the adopted kids from wanting, sometime in the future, their own biological kids?
Adoption is a really sweet and cute method of having children, but it just doesn't sit well with some people. With other people, they want their child to literally be a reflection of themselves and the family as a whole(again, this is probably up for debate, but thats what this thread is for). One can say that's probably as selfish as it gets, but then again, isn't the human race at least a bit selfish in some way and in some fashion? Haven't we at least done one selfish thing in our lives? I know thats being beside the point, but having kids isn't really too far fetched, if you think about it.
Sure, it'll increase the population. Sure, the population would grow so huge someday that finding spaces for parking lots are going to be a huge pain in the ass. But it's inevitable.If you adopt a kid, and he/she learns how to drive, they're going to fill up that parking lot regardless of whether they came from a womb or not(Andy/ShiningRaichu is going to understand this example).
I apologize for going past the point here, but as humans, we have learned(and can learn) to adapt to just about almost any situation imaginable. You have billions of people in the country of China, and yet they've learned to adapt(although how well is irrelevant, the fact that they manage to be holding their own seems fair enough to me). If the world population even hits 10 billion, we'll learn to adapt. Build bigger parking lots, Create more seats in the movie theatre so it won't be so jam packed. Do something so the world can be less claustrophobic or something.
But that's my two cents.