Concerning ethics, secular humanism is consequentialist ("Secular humanists hold that ethics is consequential, to be judged by results." [1]).
It seems also to be epicurean ("Secular Humanism frames morality as not causing unnecessary pain, harm, or suffering to humans and other animals" Zuckerman, "What is Secular Humanism?" [2]).
And, secular humanism's ethics is based on reason ([3]):
“Who are the secular humanists? Perhaps everyone who believes in the principles of free inquiry, ethics based upon reason, and a commitment to science, democracy, and freedom. Perhaps even you.” — Paul Kurtz (1925 – 2012)
It is possible that by "ethics based upon reason", Paul Kurtz meant to use reason (= logic and empirical sciences, I suppose) to foster what is considered to be moral. For instance, increasing the wellbeing of people by the application of experimental/clinical psychology and medicine.
But still, the meaning of the phrase "ethics based upon reason" is broad enough to ask the following question:
Granting the fact alluded above that secular humanism's ethics is epicurean, how does secular humanism use reason to ground the epicurean moral principle that good is pleasure (= avoidance of unuseful pain)?
Why avoidance of unnecessary pain be more reasonable, as an ethic principle, than any other thing (e.g. the preservation purity of XX thing)