Often logic is referenced as an absolute "ground" for knowledge. Fair enough. It is often pointed out that we cannot even have a conversation that makes any sense at all without the rules of logic. Certainly this is true.
But what do we mean if something is illogical? What we mean is that if something doesn't make sense, that it cannot be. And if it doesn't make sense and it cannot be, then it doesn't exist - necessarily.
For instance, a theist will rightly criticize the argument that God cannot be omnipotent because he cannot create a stone that he cannot lift. It's a bad argument because it criticizes God for not being able to do something that is illogical. God also cannot create a married bachelor. Why? Because it is illogical and illogical things cannot exist.
But morality is different, because immoral things can exist.
So if morality is the idea that a certain choice is better than another (and they both exist!), it begs the question - why is one choice better than another? Surely the only way to establish which choice is moral is if there is some sort of goal in mind. This is where moral theories come in, I suppose. X is better than Y because it will lead to greater happiness, lead to the preservation of conscious creatures, fulfill more desires, etc. etc.
The idea of morality does imply the idea of an absolute standard of good. That makes sense. But we must define what "good" means. If we do not, then our "morality" is arbitrary. For me, good means things coming together. The opposite of good is separateness. ( BTW, this does not mean that someone going off to live in the woods is doing something evil. A person needs to be connected to nature too after all. We are all made of the same stuff.)
And how do I, and we, ultimately ground a definition of "good"? I am not sure we can, except in our deepest feelings, our intuitions, about what the meaning of existence is. And one person may feel this is because God created this sense in us, a "natural law", and another person may think these deep feelings are the result of natural selection and cultural conditioning, or someone may think that both of these ideas are true. But the process of seeking morality becomes the same in both cases. We use our reason to probe our deepest feelings and educate ourselves in order to find the best course of action to satisfy the goals of these deep intuitions.