When magazines aimed at adults ask overly sexual questions of child actresses, call them hot, jailbait ect. they are sexualizing young girls. This is bad because it is an inappropriate. Both the action (the published words) and the subject (the specific girl) are real, thus the harm is real.
People online will call fantasizing about about adults in diapers "sexualizing children". What they mean by that is "promoting child sexual assault as a practice and being somewhat guilty of it yourself". This is wrong on numerous levels. Both the action (the fantasy) and the subject (the adult in a diaper) are hypothetical and not real. Second, the fantasy involves no actual children. Finally, even in a fantasy involving something that would be boundary violating in real life, that is not how that works.
That is the mechanism behind the Christian belief of 'Devil Worship' wherein by praising or giving devotion to the entity known as the devil, he becomes more powerful and is more able to influence events on Earth for the worse. Similarly the concept of 'sinning', ethical crimes comittable by both action and thought as the omnipresent god entity can read thoughts, are a huge basis for the thought crimes belief. Additionally, the demonization of sex is a well documented Christian phenomenon.
Chastising (guilt tripping) strangers on the internet for things they imagined and did not actually do is both useless in preventing real world sexual assault and very rude. It only works if
1. The target assumes "sexualization" is a dirty word by default
2. The target believes that sexual assault is a devil fed by devil worship.
3. The target believes that merely thinking of doing something to another person equates to some measure of the responsibility of actually doing it.
None of these are true of me. You cannot guilt trip me. Do not try. Instead, block and move on.