what are we supposed to say again when christians ask us where we get our morality?

[GrandizerII]

My criteria necessarily include reference to nature. They would be indefensible, to me, otherwise.

So you set the criteria, you make a judgement based on what nature shows you with respect to the criteria you set.

But you still the one who has to judge if it's right or wrong. The best nature can do is, as you said, show you that "some act leads to some consequence".