Apart from the fact that all of us live in nations that have laws that decide many of these things for us, which I am more or less obliged to obey; I would argue that I determine my morality exactly the same way most believers do — we decide for ourselves what feels right.
I would challenge any religious person who says their morality comes from their religion to say that they believe and support the morality of every little thing in their holy scripture. Let’s look at the Judeo-Christian books:
I might be going out on a limb here, but I’m willing to guess if some adult said with absolute seriousness, that they were going to kill their child because that child had a smart mouth, you would (a) recoil in horror, and (b) call the cops.
So if it’s acknowledged that we pick and choose “just the good stuff” from our holy texts, once you get honest with yourself, you can only come to the conclusion that our morality doesn’t come from the Bible, but from somewhere else (I’d posit society and philosophy). Most people see the places where societal norms and the Bible overlap and ignore the horrific stuff that is also in those books.