To play devils advocate why should we trust our innate sense of morality?
We shouldn't. Just as we don't always go with our gut, but have to use reason to figure out if our gut is right.
I think we need to make a distinction that's, unfortunately, covered by a single English word.
We all have our personal moral reactions. For instance, you might be physically repulsed and angered by the sight of someone burning a national flag and feel it's morally wrong. Or you might view it as a brave act of political speech, get an emotional uplift and a flutter in your chest from it, and feel it's morally right.
Two people from the same nation viewing the same event might feel entirely different ways about what they're seeing.
And there's really not much we can do about that, we just feel what we feel at the time, although our moral responses change over the course of our lives.
Then there's the social application of moral codes. For instance, what happens to the person burning that flag? Do we decide that one moral view of what he's doing is the correct one? And so do we jail him or pin a medal on him? Or do we really not care because it's a personal matter with no import to anyone else?
We have laws and social customs to enforce our answers to those questions.
But in neither case is it a good idea to allow gut feelings about morality determine behavior as a rule. As long as we're living with other people, we have to use some sort of restraint if we're not simply psychopathic or something like that.
So we have to make decisions.
"Yes, I feel like this, and I really don't like what's going on there, but I should put up with it anyway because that's the law and it's a useful law."
"Society simply can't allow that behavior so anyone who engages in it needs to be imprisoned or killed."
"It sure is an awful thing, but it wasn't really his fault, so he shouldn't be punished."
"We don't want to have to harm someone else, but it's something that has to be done in this case."
Since we have to make decisions, rather than go with our gut (or the gut of the most powerful person), what tools do we use to make them?
Some argue for holy books, but the results are mixed at best, truly horrific at worst, and the books all disagree anyway, plus they tend to mistake superficial customs for substantive morality.
Biology offers us a way out of the choice between that kind of option, on the one hand, and the idea that morality is entirely random and arbitrary, on the other.
Rather than looking at our feelings, we look at our species, and we figure out what the basics of human morality are, how they work, and where they can go wrong.
Seems to me, that's really our only shot at any objective basis for moral decisions.