> If I believe that "it is good to reduce net human suffering so others can live fulfilling lives" I'm not sure what evidence there is to give to refute

I think that if an argument is clear, one can always think of counter-evidence or construct at least a hypothetical experiment which can test it.

Ultimately though, my (probably controversial) answer to increasingly abstract ethical questions would be that when the goal is to find what is True then reducing suffering (human or other) as well as other ethical endeavors becomes a necessary by-product.