I think you are making too much of humanism. Humanism looks at the behaviors of the person, the intelligence or being (or individual, or soul, or spirit, or man), and divines natural law from that, from the physical expression. Natural law has no requirement to be made to be expressed physically, because it derives from a physical expression; it has no need to be made to be universal, because it is what is expressed universally in people. If there are cultures that don't reflect that universality, it's because those cultures are "not humanity"--hence the slave trade was possible--and not because those cultures don't conform. (It's also what made the abolition of slavery possible: the lifting of humanity even higher, over and above the economic necessity of a culture.)