Namaste Amir,
You have no dispute from me that different relgions and cultures have differences in their moral interpretations, but it is generally true that all cultures agree on the basic wrongs: murder, theft, adultery, rape, dishonesty.
Now it is often the case that these religions do not practice what they preach. As you have pointed out with the example of prophet Mohammed. However, the same Quran says “Let there be no compulsion in religion” “Be ready to forgive those who transgress against you” For the Muslim it prescribes living kindly and honestly, not commiting adultery and not being dishonest. Most Muslims you will talk to will say this is what a good muslim is.
The Hindu is taught the same general moral teachings: be kind and compassionate, be forgiving, do not commit adultery, murder, rape, theft etc. Very much as the Christian is. These are basic common sense morals, that even atheists share. Most people you talk to about morals will say do not rape, murder, cheat, lie, steal. So universal is this agreement it is law in most socieities in the world. This is not because of some popularity contest based on how many people agreed with a moral code authored by somebody, but independently almost every society on the planet has arrived at these basic wrongs.
This indicates something objective about morality which all societies discover for themselves. A very powerful phrase I read in a book I have says, “We do not invent the laws of nature, we discover them” Similarly, we discover moral laws. When you know a law, then you can make predictions based on that law. The most scientific study of morality has been done in Hinduism with the law of karma. The law of karma is a moral law that operates in the mind. If you have negativity in your mind, it produces negative mental states, which in turn leads to negative actions. The Hindus realised that what leads human beings to negative actions was attachments. The attachments consume us and corrupt our intellect, and with our intellect compromised, we no longer behave rationally. For example I am in love with this woman who does not love me back. I grow attached to this person, and hope that she will love me back some day. I feel a sense of lack and indequacy that she does not love me(Why not, what is wrong with me!) I start to get frustrated when it dawns on me she will not love me back. I get angry at myself and at her and my intellect become corrupt. I no longer act rationally - but I act emotionally. It is then I am in a condition to commit wrong actions - such as stalking her.
It is easy to see that behaviour like stalking another person comes from a mind in a negative state, not a positive one.
With experience we are able to identify the negative mental states and their associated mental behaviour. The Hindus identified 5 negative mental states(anger, lust, fear, jealousy, vanity) and associated with these are wrong actions like murder, gossip, hatred, theft, egoism. It is very easy to see that one who commits actions like rape and murder is somebody who is suffering immensely from negativity in their mind. A person who is positive, loving, brave, confident etc would not murder or rape somebody, because the cause for such is absent in them. This is how Hindus were able to arrive at a scientific and objective law of morality. They understood paap as action that brings defilement to the mind, and punya as action that gets rid of defilements and awakens virtues.
Thus morality is not just an opinion, but very much a law of nature. If you go against this law, nature will punish you, irrespective of whether you like it or not. One can escape human law, but definitely not the laws of nature. Nature punishes you there and then the moment you even consider acting out a negative action.