Consider this (meta-)moral principle: For some moral proposition P, if universal human obedience to P would lead to human extinction, then P is false.

Example of application: Consider "It is always immoral to have children". Obviously, if humanity universally obeyed that principle, human extinction would inevitably and quickly follow. Therefore, that principle is false. From which obviously follows: "It is moral to have children (at least sometimes)".

Now, I would propose we accept that meta-moral principle as an axiom. To me, and to many others, it seems self-evidently and intuitively true. Who seriously doubts or denies this axiom? Well, most of the people who doubt or deny it, do so not due to its specific content, but due to a philosophical objection to accepting any moral axioms at all. The number of people who are happy to accept moral axioms but would refuse to accept this particular one appears to be rather small. It seems as good a candidate to be an axiom as any other. So why not accept it as an axiom?