My gf and my best friend

My girlfriend (24) and my best friend have been around each other our whole relationship. They pick at each other all the time, she refers to him as her other bf because he is around so much. Some times when they go back and fourth it has a sexual undertone. For example she made a comment towards him and he said “suck my dick” meaning fuck off, and she said “pull it out”.

This past weekend about 20-25 of us were hanging out and drinking. Jeff (my buddy) is always doing somthing crazy, right in front of my girlfriend and like 3 of her friends pulls out his dick and starts pissing. My girlfriend told him to put it up and go piss over by a tree in the woods behind her house to follow her and she would show him. I overheard her friends talking about how long his dick was. About 30 minutes later we were inside and i saw Jeff knock an ink pen off the counter right in front of Jennifer on purpose, and she bent over with her ass on him and said “did you drop this”. Fast forward another 45 minutes and we’re all pretty drunk at this point, Jeff comes up and says “hey what if i give Jennifer some dick tonight” and i thought he was kidding and told him if he could talk her into it to have at it. A little time passes and i didn’t see Jennifer around so i asked one of her friends where she was and she said “you may want to check the bed room. So i walk back to our bed room and Jeff has her bent over the bed pounding her. I was shocked and she looked up and said “give us a little privacy I’ll be out in a minute” and he went back to fucking her.