https://en.wikipedia.org/wiki/Antinatalism
I guess this is about not having children, because... The world is so cruel and rotten that by the moment you give life to somebody you condemn them to the horrible fate of having to experience a horrible world.... So this idea is not really based on being with too many people being bad for the world, it's rather that life isn't as beautiful as you want it to be and that you would be better of if you never lived but as your parents made the decision for you that you should be born, putting you on this cruel world would be an act of evil...
Now I am not saying I agree with this "philosophy", I'm just explaining the idea behind it. If my parents decided not to have children they would also have decided for me that I should never have existed, and since I didn't yet exist when the decision was made I could not defend myself against that idea. So no matter how you explain this stuff, you'll always end up on the side of evil.
When it comes to the planet, I think there are a few other things that come to mind. Yes we are with too many humans on this planet, however, by stopping to have children altogether you can also be a killer on the planet, economy and many other things, so here too you are always on the side of evil. Also remember breeding is an instinct and we got that instinct for a reason. Also remember that back in the old days it was not uncommon that in a 7 children family at least 4 or 5 died before reaching adulthood. Now that our medical system is better that is no longer the case, but evolution is always behind on us, so our instincts still drive us in old ways. That and the fact that in some countries there is no elderly care, forcing you to have many children so you are being taken care of when you are old, as long as that issue isn't solved, we also have quite the problem. And the 1-child policy in China (only 1 kid allowed per couple) is gonna get them a very high-priced bill someday, I tell ya.
So many children isn't good. No children at all isn't good. Moderation is key. And when it comes to antinalism as a philosophy, that is just as evil as breeding if you really go into that...