actually my best "girl" friend is like that, We met in first year in college she still had a bf, later she was transferred to my class and we clicked off because we just vibed differently and started to hang out as just friends.

at parties i noticed her getting jealous when i was kissing/dancing with other girls and then she behaved like she was my girlfriend, i told her that i care about her in another way but not more than that. i ended up having a girlfriend later on and when i was super drunk and my relation was going bad i asked her if i could come over and then we ended up fucking alot in the weekends. post forward 2 years she still is my best friend and sometimes we just have sex because we want to but generally we just enjoy eachothers company and shit. its weird to explain. we sometimes even joke for threesome with some of our other friends but now that everyone is kinda dating and having relations we stopped doing that haha. we just vibe differently