Contactarianism orgininated as a political theory, and only later devoloped into a theory of morality. Social contract that free, equal and rational people would accept as the basis of a cooperative life together. Actions are morally right if and only if they are permitted by rules that free, equal and rational people would agree to live by, on the condition that others obey these rules as well.
Everyone wants to be ate the top of the heap, and only a few can make it there.
If we are truly rational, we will each agree to curb our self-interest and cooperate with one another. We will do this conditionally. If we all stop trying to get the better of each other, and instead agree to seek a little less for ourselves, then we'll all be better off.
Basis assumtion: we're all motivated by self-interest, this is rational.