@MysticMoonlit you overthinking it. She just was raised like in the glass box (girl from the rich family with servants etc), they protected her from "bad influence" (this theme is not rare in the manga), and probably convince her, while raising, that boys only want her body or her money or her influence (tried to "protect her" this way, instead she is just like little girl in the sex and relationships questions).
And probably there were few from before, that proved those statements.
But our MC is just normal guy. And yeah - he stand out in her eyes. Other guys afraid her reputation (obviously not without a reason), so she is thinking that they all alike, and treating them as well. And about him she thought too, but he was not afraid of her, and caught her attention by this.
Man, it just simple as that. Girl in her bloom meet a guy, who can confront her. First reaction "who the hell he think he is?", second "wow, he can stand for him self", and then natural "he is not like the other boys I met before". Add some hormones, some natural teenagers curiousity - and you have your horny girl who has her first crush.