Someone asked this question below, you can see it on Quora here
My reply is below
According to some libertarians, the idea of a social contract is invalid. If so, on what basis do they argue that I can’t steal, as I have never explicitly committed myself to respect their property?
The social contract is a cudgel that says one MUST act i.e. forcing one to engage in an activity one would otherwise avoid (eg taxation toward some putative communal good).
Not stealing compels no action. Not stealing is inaction. Not acting costs nothing. Acting carries a cost (opportunity cost e.g. I want to do x but the social contract forced me to do y). So forced or required action not explicitly agreed to carries with it an outside imposed burden.
The social contract is invalid not because it is imaginary but because it imposes costs not agreed to. So even if you believe basic tenets of morality (such as theft is wrong or murder and rape are wrong) are imaginary, it costs nothing to not engage in those activities. Indeed it is advantageous to not do so insofar as it creates additional opportunity to engage in other more productive activities.
Social contract limits choices. Morality expands them. Perhaps both are imaginary constructs but positive obligations are inherently more limiting than negative obligations.