why cant they just be normal
Characters who are "normal", which is a very subjective statement by the way, are not that interesting. I wouldn't write a story about Kyle who works at a factory, then goes home and does the same thing every day. That's pretty normal, but would make for a very boring story.
Sarah didn't seem like the kind of girl who goes out much, so really finding out she was a slut
Because people change, she obviously realized she couldn't keep doing whatever she was doing, so she made a change.
Feel bad for Mark, probably got some STDs
If this is what happened to you, then I'm sorry, but there's been no indication that this is true.
Why did we even need that information
Because it made sense to have it there. What did she do before? How did she manage her feelings toward Mark? Since she's otherwise not very good at managing her feelings, it's probably something that's not very good for her in the long run. She's that type of person. Even her relationship with Mark is questionable.
She says she got a reputation of being a slut, not that she actually was one. People getting a reputation unjustly is of course a thing that never, ever happens, you just kinda assumed it was true, as did Mark.
Sorry you didn't like it, hope you find something else to play instead.