The implication of the term "sexualize" is to turn something nonsexual into something sexual by interpretation. It's like anthropomorphism, but for sex. (Perhaps a less politically-charged term, like erotomorphism, would be better). For example, the movie Sausage Party sexualizes supermarket produce, by drawing the produce in a sexually suggestive manner, giving it sexually-charged dialogue, and involving it in sexually explicit activities.
Is sexualization a bad thing? Not necessarily. If you've been taught to consider it a moral imperative to avoid sexuality, because, e.g., it's indecent, or you view it as a sin, then perhaps you might think so. But from a sex-positive perspective, making something nonsexual sexual is like having a cup of tea, and then adding a spoonful of sugar to it. It makes things better!
But can sexualization be problematic? I suppose it could, but only when imposed upon another person against their wishes. Now, with people, it's tricky, because people are sexual beings. The term sexualization, as I have found, is misleading - you can't sexualize a human being because human beings are inherently sexual. But I suspect that this is not how the term is typically intended to be used. It's not necessarily the person who is being sexualized, or even their body - it's the situation. I feel like we'd be better off replacing the misleading term "sexualization" with something more context-specific, like "unwanted sexual attention".
When a girl decides to dress up as a sexy witch for Halloween, she is (most likely) consciously choosing to be sexy. You could argue that she has sexualized herself by choosing what could be described as a sexualized Halloween costume. Yet, the fact that she has consciously chosen to attire herself in a sexualized fashion does not, nevertheless, mean that you are entitled to, e.g., sexually harass or assault her. See: SlutWalk, and "skirts are not consent".
However, we must be careful to distinguish between sexual harassment, and mere acknowledgement of sex appeal. If a girl dresses in a sexy outfit, you are absolutely entitled to appreciate it as such. You just need to know the line between that and harassment (and I acknowledge that, unfortunately, feminism has blurred the line to an unforgivable degree). I think this is where the terminology of "sexualization" falls short, because it doesn't distinguish between positive and negative sexual attention, and you can't fault a person for becoming frustrated when a girl wears, e.g., what amounts to lingerie in public, and then complains about people ogling her body.
Obviously, what constitutes a sexual cue can be subjective, and there is room for misunderstanding. The girl who picked out that costume with the skirt and heels could have simply thought it looked cute, not intending to be the subject of catcalls. And this could be true of any outfit - even everyday clothes. You have eyes - you're allowed to appreciate all the beauty you see in the world. (This is where cries of "don't sexualize me!" fall on deaf ears). But use common sense. Be polite. When giving girls attention, try to read the situation. If they're clearly not interested in fending off flirtatious advances, then leave them alone! You don't want to make them uncomfortable. That only teaches them to dress more conservatively next time.
And by all means, don't tell them to change because you're the one feeling uncomfortable. "Triggering" your sexual desires (oftentimes inadvertently and unintentionally) does not entitle you to anything. I wonder, sometimes, if I'm coming from an unusual place. In my experience, the world is filled with erotic stimuli. And I love it! It's like going through life with a constant buzz (not necessarily in the sense of being physically aroused, but mentally stimulated). I'm not frustrated; I'm delighted! Yet I wonder if it's because my libido is atypical. Do men really have to have satisfaction every time they're stimulated? (And is it not enough for them to remove themselves to a private location and take care of it alone?).
Maybe men have been trained to get what they want, but to me it seems like entitlement. Can the solution truly only be one of these two evils: 1) a world where women must keep their bodies covered at all times, so as not to entice men, or 2) a world where women must submit without resistance to men's desires at any time? Neither one of these is a world I want to live in. And both of them discourage expressions of female sexuality. They're really one and the same. Because what woman wouldn't cover up to avoid the risk of enticing a man who is legally entitled to rape her, and what man would be criticized for punishing a woman who violates his society's imperative for her to cover up? I prefer a more progressive alternative, where women are free to express their sexuality, and men are responsible for their own reactions to it.
Let's end this with an example. It's a hot, summer day. A girl decides to put on shorts. Now, she could wear baggy, knee-length shorts (or longer) like a lot of guys wear. These shorts are not particularly sexy. They're not skin-tight, and they're not designed to expose an excessive proportion of the legs. Or, she could pick out a pair of short shorts that are so popular among girls these days. I'm not going to try to pretend that these shorts are not what you might call sexualized. Yes, they're just shorts. But they're shorts that expose a lot of leg - particularly the thigh, which is in closer proximity to the sex organs. Yes, it's just a body. We all have one. But we're also designed to find them appealing (accounting for taste).
Yet, we're not cavemen here. Laying eyes on a juicy thigh is not a mindless excuse to initiate coitus. At the same time, expecting people to pretend that it's not a desirable stimulus is inhuman. We must be able to reach a middle ground. I'm a nudist, so I've had experience standing before an exquisite specimen of naked glory (and while they may be few and far between, that does not diminish - rather, it may actually enhance - their spectacle). I find no justice in denying the effect it has on me. But at the same time, I am able to comport myself like a civilized gentleman. This is the standard I hold for mankind. Please tell me it's not too high. Because the alternative is a bland world in which women, fearing rape with no repercussions, are compelled (by law and society) to wear only formless, grey jumpsuits. Don't send me to that dystopia. Please!