Here's the basic line of thought:
Men occupy a more powerful position in society due to the generally patriarchal structures. Women occupy a less powerful position than men, even when a particular women holds more overt power (e.g., a woman that's a CEO). As a result, sexual relationships between men and women always have a power imbalance; that imbalance of power means that women can never really be consenting, since there's always some form of 'threat' involved. A woman that believes she wants sex believes that way because society has conditioned her to be that way, rather than that being something she chose in a vacuum.
And theoretically, this is all true, kind of. But it also isn't, because that would mean that women can never have any agency over their own body or their own sexual choices. ...Unless they "choose" to be lesbian, which isn't actually a choice at all.