A made-up quote falsely attributed to Catherine MacKinnon. While this isn't a real quote, there is an argument akin to it.
The defense I was first presented for it was that because our society has been built up by men in a way that all women feel worthless and the only way to have any value is to serve the mighty male by having sex with him. Since the oppressive way which men have created out society forces women to have sex in order to have any worth, all sex is rape.
This is a simplified argument, and lacking in nuance. I think a defense closer to what would typically be made is that we're trapped in an institutionalized power-dynamic that makes entirely free consent difficult to achieve, if not impossible under the current patriarchy.
Use of the absolute "always" and using the term "rape" makes it sound to me like something someone would falsely attribute to feminists in order to them as too damn radical. But we have a radical, pervasive social problem, and if I say "pervasive gender roles and institutionalized powder-dynamics make sex inherently coercive", it's the same thing, but now rings true, or at least doesn't kneejerk you into disagreeing.