Story of my girlfriend's falling out with her best friend, made from my porn account because they both know my real account.
My girlfriend's best friend was really hot. But always wanting some attention or affirmation that she was hot, so she was really outwardly sexual. It was uncomfortable for my girlfriend for a while because she would push the boundaries of what was okay to do with me and talk to me about. I pretty much took the "no comment route", I didn't encourage her behavior or discourage it. She would worm her way in to cuddling with us, show us her nude photos (which were super fine), get partially or fully naked, freely change with me in the room, we even slept together whenever we stayed out. In fact she did overtly sexual shit like this so often my girlfriend became accustomed to it. One time I showed up at her house and my girlfriend as well as her two best friends were butt ass naked and stayed like that the whole time I was over. It was tight. So this just kind of became our relationship with the three of us. All this being said I love my girlfriend and would never cheat on her, nor did I think I had a chance. However...we all banged a couple times in the span of 2 weeks, it was awesome each time, not so tight like 10 minutes after though. We weren't even drunk. We pretty much talked to each other less and less afterwards. It made it weird. It's funny how crossing the boundary from being sexual to eachother all the time to actually acting on it completely changed the dynamics of our friendship. My girlfriend and I haven't talked to her in 2 years. She was kind of annoying anyways, in hindsight I'm okay with how stuff turned out, at least my penis is. For real though my girlfriend and are perfect, this never made it weird between us.
TL;DR my girlfriend, her best friend (hot), and I had a threesome a couple times and stopped talking.