It really sounds like you have a mental model of the 'social contract' concept that isn't aligned with what it means for everyone else. To be clear, it says that we (possibly tacitly/implicitly) agree to give up some freedoms to the government in exchange for their governance/defense/etc. It has nothing to do with the enforceable contracts and things you sign. It just frames the relationship between government and constituents in terms of an agreement.

So if we give up certain freedoms in this agreement (such as the 'right to not be taxed'), then it's not unjust - we have (tacitly, implicitly) consented to this. It's true that you the individual might not have consented, and might not have the means to leave if you don't like it. But the collective is consenting, and can change the constitution if they change their mind.

So taxation is not inherently unjust if the state's constitution says it can tax you by definition. You might think the social contract that we're using is bad, or that the constitution is bad, and that's fine, and you can change it democratically if enough people agree with you. But 'unjust' doesn't really work.