We sit in font of computers the whole day. The disciplined ones runs when there is no immediate danger or prey. We don't attack or kill people when they make us angry. We don't have sex with everything that has a chance to bear offspring.
Now the main problem is that if you reduce your CO2 emission by 80% and you and all your offsprings have two kids and each consume at your reduced level, in just two generations, it would been better if you didn't have kids. Two generations after that even at the reduced consumption, you (by having kids) contribute to CO2 emission by a factor of 6 if you have just consumed all the meat and didn't have kids.
Geometric progression is a nasty nasty thing.
Two kids per couple isn't enough to cause population growth.
Two parents having two kids leads to the exact same number of people on the planet after the parents die.
We only see population growth with 3+ kids per couple, and that's becoming increasingly uncommon in developed countries.
Absolute immortality isn't a plausible scenario, but it's important to consider that as longevity increases, the reproductive rate that leads to a stable population must go down.
Whilst human population can't continue to grow on this planet indefinitely, there's plenty that can be done to push our societies in a direction that works with nature. That should be our focus.
To give one example of how working with nature benefits us all, check out this talk by William McDonough:
We haven't done all we can to protect the environment. We haven't even tried that hard. It might be easier to reduce our personal impact to 10% than try to reduce the world's population by the same level.