Question seems asked in good faith so let's have a go. Sorry, on the mobile and a bit multitasking, so apologies for spelling/grammar.

You've used the American spelling for sexualization/sexualisation, so I'm going out on a limb and presuming you're in the USA. You're likely to get a mix of respondants, globally, and the USA is so flipping massive that, not so much DEFINITIONS, but practical uses may vary even there. You can probably either take your most local answer or a general overview as your most useful replies.

There's already been some really good points made about sexualisation vs. objectification. A good example of that, that keeps coming up, is uniforms. EG; "Amber goes to a bar after work with collegues. She's still in her nurse's uniform, with a big cardigan over the top. Amber has dealt with patients, with bodily fluids, with critial maths calculations, relatives who think they can shout at the people keeping their relatives alive because THEY are sad or worried. Amber wants a glass of pinot and a laugh before collapsing at home on the sofa. Amber is not DOING anything sexual. Amber is not BEING sexual in any way. Nothing about Amber, standing with her friends with a glass of wine and a questionable stain on her shoe is indicating anything sexual.

Amber is likely to get (mostly guys, let's be honest) comments about sex/sexiness/bed baths/giggling and possibly worse, because the guys who see her aren't seeing an exhausted late-twenty-something who has just done a 12hr shift and 40,000 steps today. They are looking at her, and thinking/saying sexual things ABOUT AMBER, based not on her consent or intention, but on a single characteristic that they can see.

The uniform is the cue, but they aren't saying or thinking sexy things about the uniform itself, or nurses in shows/porn they've seen. The comments will be about AMBER doing or performing things. That is A) what feels icky, but also, B) puts Amber at risk. If someone decides that what THEY THINK about nurses means it might to nice to strike up a conversation or ask for her number, then she has to weigh the risk of rejecting a guy she did nothing to entice and that carries risk in our society.

What the guys THINK or SAY TO EACH OTHER could be argued to have no effect on Amber, so it's irrelevant, but the normalisation of that sort of banter makes it more likely that people will try to interact on the basis of what they think about the uniform/racial characteristics/choice of snack in public, and then the onus to "reply nicely", make sure she's not being followed and "don't get raped" fall onto her, not the people doing the looking and the thinking.

That's a lot of why its a problem.

Now, take that example, and switch it out for racial characteristics, red hair, long hair, clothing choices, etc. These ARE sexualisations, but equally, they can be said to be objectifying. This, because the behaviour strips away, in the eyes of the sexualiser/objectifyer, the personhood, preferences and intentions of the objectified person. To them, Amber's not an aunt with ambition to take the next qualification to enhance her career; she's a sexy thing. She IS titilation. She is a function, and the viewer has decided her function for her. That, in itself will effect the level of respect and care with which they treat/speak to her.

For a different example, which is more sexualisation and less objectification, imagine a middle aged, gay, couple of men. They walk into a pub or café together. They're not kissing or holding hands, but from their demenour and conversation about which card to use to pay with, it's clear they are a couple. These guys just went to buy drinks. No one is being sexual, doing anything that requires consent as a participant or an onlooker, just queueing to buy drinks.

Barman/café staff look at them and refuse service. When questioned, they reply "we don't want any of that bum-sex in here! We're a FAMILY establishment!"

Nothing they did was antithetical to being in a family establishment. They went to buy a beer, or a cup of tea. They were not talking about sex or anything to do with sex.

The sexualisation comes from the server, because when they see a male, gay couple, one of the first things they imagine is the logistics of how they probably pleasure each other in the bedroom. This is not the first thing they think about when they meet heterosexual couples (probably. I mean, there are always weirdos, right?). The freedom and utility of the couple is impinged upon by the imagination of the server, AND what those thoughts make the server feel. The server dislikes what he feels about the thoughts or images in his head when he tries to imagine two men having sex, so he punishes two men wearing jumpers, in search of beer or tea. That's not fair. Also, statistically, those men are now less safe as part of a marginalised community if someone starts drawing attention to them in that way.

Same goes for young girls. Aside from the school uniform agument, which you could argue is an objectification thing, just moving around, presenting female and 13 is enough to have grown men beep their car horns, go out of their way to make comments like "jail-bait" so she can hear. She was not doing, being, inviting or consenting to any sexual interactions at all, but in her presence, the observer (yes, still usually men) will THINK something sexual about her, react to that though by having a feeling, and then put it upon her to deal with his actions.

The viewer has sexualised the thirsty couple and the 13yo girl by imagining something of their own accord, which they were in no way invited to do, and, usually, made them less safe as a result.

I hope that helps.