If morality is to be objective then it can only reside in something external to human construction.

Anything that depends on you to define is by definition subjective. And your opinion can't be truer than someone else's opinion. Your moral standard is no better than anyone else's moral standard.

Where else do you get a standard of Good? Your opinion? a society's opinion? a majority's opinion?

You ask me to spend time looking into the reasons atheist behave morally, but which reasons should I look into? if there is no standard, how can I know which one is better than all others to know which one to emulate? and if it's only your opinion how then we judge anything good or bad?