Humanism is a non-religious ethical outlook based on an interest in human affairs at the human scale. It is not a doctrine or a set of rules; it is a starting point, its founding idea being that ethics must be based on the facts of human experience. For some, the result of thinking for themselves about ethics might be close to a conventional moral outlook; for others, the result might be less conventional. Either way, there are just two constraints: that one's choices must not be aimed at harming others, and that one must be able to make a solid case for one's outlook if challenged by others.
One great flaw in religion-based moralities is that they are not thought through and chosen on the basis of individual responsibility, but are imposed from outside in a one-size-fits-all way. As a result, religious morality too often cuts across the grain of human nature, distorting it and crushing its natural impulses, not least as regards sex – always an inflated subject of interest for religious moralists, who throughout history have been frightened of it and bent on limiting it as strictly as possible.
As this shows, humanism is a response to Socrates's invitation to live the chosen life, rather than a life prescribed by doctrines inherited from the traditions, especially the religious traditions, of whatever community one happens to be born into. Religious moralities assume that there is one great truth and one right way to live for everyone. Another great flaw with religious morality is that it says if you do not obey, you will be punished. The threat of punishment is not a logically adequate ground for moral behaviour, even if it is prudent to avoid punishment by behaving as ordered. Unless one's moral outlook comes from being thought-out and chosen for oneself, it is at best an imitation of morality, at worst a subversion of it.