I want to write about this in more detail, but I came across the linked article recently and I thought it did a good job of illustrating one key point.
Objectification isn’t “finding someone attractive”, or leering at them, or harassing them, although it can play into all of those things.
Objectification is literally seeing someone as a non-human object, like a cube, or a climbing frame.
Objectification is when you only acknowledge a woman when she has placed herself inside a plastic “object” shell created out of makeup, restrictive clothing, performed feminine behaviour and is in some kind of vulnerable situation.
What’s being acknowledged then isn’t a woman any more, it’s the exoskeleton/cage of “woman”.
If parts of a woman show outside of the cage - for example, stretch marks, or un-made-up eyes (made-up = fantastical = not real), or god forbid a defiant personality, it will either be forced back into the cage (oh, she’s so feisty! kitten got claws!) or, the woman having “failed” to present as an object, she will not be acknowledged as a “woman”.
Mary Daly* writes about relations to fetish objects in Gyn/Ecology that a fetish is a sexual relation with an object. When we look at what culture recognises as a “woman”, we can see that culturally normative male heterosexuality is a sexual relation with an object, that is, a fetish for the “feminine”.
Daly also calls this necrophilia, a sexual relation to a thing which is not alive, which in its course requires the literal or figurative death of a person (the woman) so that her body may become the non-human fetish object.
This is why objectification is one of the key processes of patriarchy, and is so enabling for rape culture. It destroys women and turns them into non-human things, which then of course become un-rapeable, because a climbing frame does not have boundaries to violate.
It’s also why claims that men can be objectified are often pretty tenuous. There was a post I bumped into a few days ago which I can’t seem to find now which gave an account of a semi-genuine case of this, where a man was treated like a woman after he took his shirt off, I think in a music video. Over time he felt more and more like the audience wasn’t seeing him, just a plastic (my word) torso, and had (or came near to?) a breakdown. Now imagine the effect on women who are conditioned to this from birth.
EDIT: This is the article I meant. Thanks, orlikeawhale!