I've only skimmed the last few posts, so I don't know if this has been mentioned, but the thing is most Brooke fans WILL admit she was slutty, and it's kind of what made her the Brooke we fell in love with in a weird way. It's the same as Peyton fans liking her for being a bitch (FYI not calling her one, just repeating what I hear), or Haley fans liking her for speaking her mind. It's not the world's best quality, but whatever.
The thing is, the other characters don't have their pasts brought up in the same way. Peyton's past was not mentioned, even when she was reprimanding another character about something similar. Haley's past was not mentioned when she was specifically talking about the value of marriage, WTF at that. Lucas's past is never ever brought up, when he does something stupid it's ignored like 2 episodes later, he's never accountable for his actions, but that's another issue. Nathan's past is brought up sometimes, but that's because the situations with third parties keep happening again and again for NH so it's a bit relevant (although I can see it getting annoying). With Brooke, it's little slut comments for no reason at all, I don't see why she gets the brunt of the "reminding" when none of them really had that flawless of a past.