"More humanism, relativism and tolerance with man as the measure of all things?"
I can't name a single totalitarian regime, war or genocide that was perpetrated in the name of tolerance and humanism, that's for sure.
In the name of religion? I think historians have lost count.
Humans are the moral yard stick because, as sophonts, we are moral agents - and currently the only known moral agents. Perhaps there are other sophonts in the universe, or perhaps even here on Earth (ravens, elephants, chimpanzees, dolphins and dogs all show remarkable levels of intelligence, and evidence of altruism, though to what degree they are comparable to humans is still hotly debated), but for now, we are alone, and have only ourselves as the source of human morality.
There is still widespread debate as to whether God exists or not, but aside from a few solipsists, I don't think anybody is debating the existence of Homo Sapiens sapiens.
Therefore, with man as the only moral agent that we know for certain exists, the only measuring stick we could logically use - the only reasonable standard we can use - is that of man.
Actually, I don't see why theists have a problem with human-centered morality. After all, religion is man-made. Religion is a philosophy that was concocted by humans for practical purposes (explaining then-mysterious phenomenon, preserving knowledge via tradition, promoting social cohesion and codes of morality). Even though we have come up with far better philosophies and schools of thought, religion can still fit within the humanist paradigm. But not fundamentalist religion. Fundamentalist religion runs counter to humanism in almost every way.
(sorry for the essay-length comment - I have too much spare time and should really just start a fucking blog to vent my useless philosophical ramblings)