I'm scared to tell my friend she's gone too far

I've always been the biggest prude around, never kissing boys or anything above that.

Well, when my best friend hit 8th grade, she got a boyfriend. At first I was like okay great for you, but then her boyfriend started not to like me because I was taking up all of Karlie's (my friend) time, so I started to dislike him.
On Halloween night, I was sitting on the floor in her "hang out" room while me, her, and her boyfriend were watching I Am Legend. I turn around and she and her boyfriend are under the blanket making out. It literally freaked me out, and I wasn't sure why, but it was just so weird to suddenly see my best friend being smothered by this boy she had been "going out" with for a few weeks. Throughout the rest of the movie, it was really awkward for me. I would hear her giggles and the sound that their lips made when they parted and I wanted to close my eyes and shut out my ears. I had never kissed a boy and even though I had tiny text-message crushes before, I wasn't ever used to having a boyfriend.

At the end of the night I went home and I texted Karlie saying that she and Sid kissing so heavily made me uncomfortable, and she apologized, saying that she felt bad I had to sit there and watch it. I was only in 7th grade, but I was mature for my age, so I typically hung out with older kids. I guess Karlie thought that I was supposed to know that hanging out with older people would bring a lot more mature things, especially, boys.
Karlie would never stop talking about Sid. When we would have sleepovers, she'd talk on the phone all night to him, leaving me sitting in the corner reading a book and thinking, "Gigi, just wait a few months. They'll be over."

Karlie was always the strongest Christian I had known, but her commitments started slipping a little. She would always get a little too much into kissing. Sid (her boyfriend) would kiss down her neck and collarbone and she would tell me all about it. It made me a little uncomfortable, but I had always wanted to know what it was like to kiss a boy.

One day, Karlie told me that she was going to marry Sid. I rolled my eyes and thought, "Yeah, lets see how long THAT lasts." She and Sid talked about how many kids they wanted to have and that whole enchalada. But, then one day, I asked Karlie if she still held her commitment to save sex until marrige. There was a pause on the instant chat before she answered, "Idk." It made my heart race because I knew she and Sid had been doing things. I don't know if they went full-on sex, but they'd lay together under a blanket and wrap up into each other and kiss.
I always thought they were getting too serious at such a young age, and I tried to tell her, but whenever I do, she gets mad at me and tells me to back off.

What do I do?

Me and Karlie keep getting into fights about this and recently we had such a big fight that it nearly ripped all the threads of our friendship apart.

I don't want to make my friend upset but I don't want to see my friend break her commitment.
I'm even more afraid that she'll get pregnant or get an STD or something.

I just wish that Karlie would stick to her commitment and save sex for marriage, but now, her commitment and her thoughts have changed.
Can anybody help me?