Define "unequal".
I gave you a great example in post 95.
Unequal means they don't play by the same rules.
Communist countries are inherently unequal because the government owns the means of production. We tend to assume that when we buy goods from a company, that they will eventually directly or indirectly buy goods back from us. That's not the case with Communist China. They aren't buying goods, they are buying the means of production and debt.
Countries with nothing to offer but labor are inherently unequal. It only makes sense to trade with such countries as long as your own economy is at full employment.
Inequality can also result from differing norms of social and environmental protections. China doesn't care about worker safety or environmental protections, so it gives them yet another cost advantage. Free trade then undermines the protections you've engineered in your own economy, by driving your firms out of business since they have to compete against firms that don't have such rules.