I read from here that for an A-class ranked mission, Sakura teams up with Kakashi, her sensei, and according to the requirement of the mission, she loses her virginity to Kakashi.
Is this story true? Had it actually happened in the manga?
Canon information about Sakura does not anywhere mention her losing her virginity to Kakashi, nor her being sexually-involved with him in any form. See some of the following profiles:
Same goes for Kakashi profiles:
Since the event did not occur in manga canon, no page reference can be provided. Sakura's virginity, or lack thereof, is not unmistakably specified in canon. In manga chapter 700+1 titled Uchiha Sarada, it is evidenced that Sarada has believed Sakura to be her biological mother and Sasuke to be her biological father, so it implies that Sakura lost her virginity at some unspecified point (unless she used IVF technology). However, Sarada subsequently questions whether Sakura is indeed her biological mother; this is debated by fans in threads such as here, here, and here. Sarada's doubt about biological ties to Sakura means that canon has not uniquivocally defined whether or not Sakura has ever been sexually active or pregnant.
Duty Before Honor is an English-language fanfiction written by SilverShine with a Kaka/Saku ship (a.k.a. pairing). The reader reviews for the fanfic indicate that the premise was created to be an AU (alternate universe) rendition of the characters crafted by SilverShine. Stripped Bare is also an English-language fanfiction, written by Purple_Panic (formerly known as J-Pop Princess); its reader reviews are here. Fanfiction.net and Archive of Our Own are both fanfiction repositories. As such, they do not feature canon material and are not intended to serve as a source of information about canon; in contrast, fanfiction and doujin works tend to create content that is original or intentionally contradicts canon. Kaka/Saku (written as カカサク, or カカシ×サクラ) is not a prominent ship in Japanese doujinshi.
Well, Kakashi has saved girls' life rapidly over the years like Rin. He rescued her, but later on, she made him killed herself.
If Kakashi had intercourse with Sakura, that's like a teacher having intercourse with a student. If she was over 18, it's fine, but it makes people worried because in Boruto, there is a kid that is like Kakashi but with brown hair and pink and white hair, making a brownish rose color. So they could be parents.