Simple. If you don't believe the bible or in God, then there are NO absolute moral codes and each individual can make up their own. On the other hand, if God actually does exist, then so do His laws and morals, in which case, you use His laws as the guide.
Only two real choices, God exists, and therefore so do His standards, or He doesn't exist, and you can do whatever the fuck you want.
Guess which view the leftists typically take?