"If one of the partners decides to breach the contract, it may be a shitty thing to do, but it is not immoral, there was no violation of the self-ownership of the other person, only a breach of contract."

I'd call that immoral, according to my moral code.

The fact that you disagree means that your moral code is different.

Because I've seen evidence that people often perpetually disagree on right/wrong, I'll say that neither of us is objective right.

Your moral code is made up by you, and mine is made up by me.

That's why you don't agree with me, and I don't agree with you.
Nothing is wrong with this.

It just means that morality is subjective.