The answer is that consent isn’t a binary, it’s actually a complex gradient. We all acknowledge this when we talk about power dynamics involved, we are talking about a sort of half-consent.

Marxists also acknowledge consent in things like work and economy is not binary. The Libertarians who are obsessed with contracts and voluntarism believe there is a clear binary of consent, where if you sign a contract you agree to everything. They ignore the complex gradient of coercion that comes into play from desperation for employment, desperation for housing, corporate domination of the legal system, etc.

Actually reviewing every single sexual relationship to determine all of the factors involved including age, relation, wealth, traumatic pasts, cognitive abilities, sobriety level, etc. is an insanely monumental task for any government. One that would involve massive invasions of privacy as well.

So to function, we have to sort of have a line we draw in the sand where we say consent in binary. Otherwise the state could not function and would be overwhelmed by this complexity and scale (hundreds of millions of relationships to review).