This isn't exactly right. There is the equal protection clause of the Fourteenth Amendment.
Yes, I'm aware of the 14th and this passage could be considered a form of equality too:
Article 1 Section 8:
The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United StatesWhat does this mean? It's fine to tax the pants off high income guys in Iowa as long as you tax the pants off high income guys in Nebraska too but low income people everywhere don't have to pay a thing? They get to ride for free? Is this equal? I suppose it depends on your point of view.
The point I'm trying to make is that if private citizen A made an agreement with private citizen B, private citizens C and D shouldn't be able to have the government rewrite the terms because they don't think it is equal.