Raja Halwani in Aeon:
The 18th-century philosopher Immanuel Kant believed that human beings tend to be evil. He wasn’t talking about some guy rubbing his hands and crowing with glee at the prospect of torturing an enemy. He was thinking about the basic human tendency to succumb to what we want to do instead of what we ought to do, to heed the siren-song of our desires instead of the call of duty. For Kant, morality is the force that closes this gap, and holds us back from our darker, desiring selves.
Once desire becomes suspect, sex is never far behind. Kant implicitly acknowledged the unusual power of sexual urges and their capacity to divert us from doing what is right. He claimed that sex was particularly morally condemnable, because lust focuses on the body, not the agency, of those we sexually desire, and so reduces them to mere things. It makes us see the objects of our longing as just that – objects. In so doing, we see them as mere tools for our own satisfaction.
Treating people as objects can mean many things. It could include beating them, tearing into them, and violating them. But there are other, less violent ways of objectifying people. We might treat someone as only a means to our sexual pleasure, to satisfy our lust on that person, to use a somewhat archaic expression. The fact that the other person consents does not get rid of the objectification; two people can agree to use one another for purely sexual purposes.