Problems with girlfriend and best friend?

[Gordon]

So, my best friend is a girl--let's call her Emma. Emma and I spent all last week together because we were both on holiday. One night she persuaded me to sleep in her bed with her, I figured nothing's going to happen so just as long as my girlfriend (let's call her Melanie) doesn't find out it's ok (a while back I slept in the same bed as another one of my female friends and my girlfriend asked me not to do it anymore, which I can understand, especially since we always sleep in our underwear--me and these girls are close but nothing ever happened between us; they're like my sisters).
Anyway, the next morning, when I was still half asleep, Emma took a photo of us in our underwear in bed and put it on facebook... It looked really bad.
Melanie saw it and freaked out... She told me she was angry that I'd slept in Emma's bed with her behind her back, and asked me to not mention Emma until she'd calmed down. That made me angry so I badmouthed her to Emma and said some mean stuff which I regret.
After that, Emma sent my girlfriend a rude message basically telling her that she's crazy and should see a psychologist for trust issues, that she should let me sleep next to her whenever I want, that she's known me for longer than she has, etc., and also told her the stuff I said about her when I was angry...
My girlfriend is really upset, and she wants me to tell Emma that she crossed the line and that I won't be sleeping with her in her bed any more (but that she's fine with us still being friends, just so long as if I stay over I sleep on the couch or something--she knows I didn't cheat). Emma thinks she's manipulating me and that she's being unfair.
I feel like I'm having to choose sides...
I don't know what to do...
Please help.