They both think that it's irresponsible to add to the planet's population at this point in time.
I understand and respect their decisions, although it has also saddened me - no grandkids! (sniff)
I think that if I were in my 20s today, I would probably not want to bring any children into this world, either.
Not just because of climate change, also because of the current worldwide political and economic climate. Why bring children into a world ruled by oligarchs, when those children would have nothing to look forward to except serfdom?