If it really makes you uncomfortable, tell them just that.
And as for story suggestions, I would love it if you read my newest original Story of a Girl - but be warned, it's a bit graphic.
As for stories by other authors, I recommend... http://stories.mibba.com/read/210566/Eulogies/ <-- It's creepy, and awesome. Seriously.
http://stories.mibba.com/read/165851/Living-A-Teenage-Tragedy/ <-- This one's finished, and it's absolutely stunning. It's one of my favorite things that I've ever read on Mibba.
http://stories.mibba.com/read/96223/The-Acoustic-Cafe/ <-- This has to be the most original thing I've ever read on here, and it's just amazing. It makes you want more after each update, y'know?
http://stories.mibba.com/read/212573/You-Need-Me-Bitch/ <-- It's only got its first chapter up, but it's insanely well-written. But it actually spells out bitch, so make sure to do that.
http://stories.mibba.com/read/203459/The-Runaway/ <--I just found this so insanely intriguing, and I like where it seems to be going.
Tell your friends it makes you uncomfortable. If it doesn't stop... maybe you need to stop hanging out with them for a while? I'm not sure. I'm the same as you, very passive, I avoid confrontation like it's the plague. But in that situation you really need to say something. It's not right them saying that stuff to you (not when they're so much older)
And... I have a story that's nothing like what you mentioned, it's kinda girly and chick-flicky (kinda) but you should read it anyway. It's about a 15 year old girl who has plenty of teenage drama in her life (ie, idiots, crushes and an insane family)
http://stories.mibba.com/read/197893/Claires-Chronicles/ (delete the gap that Mibba puts there... or just go to the tories part of my profile and click on 'Claire's Chronicle's')
all my other stories are fanfic, so I wont recc those (but wont discourage any peeking for curiosities sake, haha)
Maybe they will understand?
But if they don't,
Perhaps you should distance yourself a bit.
& I have a few stories you could check out.
My favourite one at the moment is;
I Wanna Scream ''I Love You''
It's all from this girl, Layla's, diary.