I actually stopped a catfight between my ex-gf and gf (now my wife) on my birthday. It was a split second decision. I think I made the right choice, but I have moments of doubt.
When I was in Basic Training, one of the guys told us a story about how he met his wife. His girlfriend had a surprise party for him on his birthday. Nothing big, just at her apartment with a large group of their friends. Her Friends and His Friends. Had a great time. After the party people cleared out except his girlfriend, her friend, and him. He had never met the friend before. They were all pretty drunk and apparently all very horny. The ladies got him to the gf's bedroom, undressed him and undressed each other. He was thinking he was getting a threesome for his birthday. Ladies were taking turns giving him a blow job, but they started to struggle with each other trying to take his cock away from the other. The struggle intensified as they both were trying to mount him first. Said they were literally kind of wrestling on top of him. It was kind of rough and uncomfortable, but sexy at the same time. He didn't know what to do so he just laid back and watched it. Eventually the friend overpowered the girlfriend and pinned her on the bed. He said she whispered something to his gf (never sure what), gave her a shove, and went to claim her prize. Said it was the best sex he ever had. Never did have sex with his gf that night or after that. Said after that the gf seemed to deflated, couldn't look him in the eye. Less than a week later, the gf dumped him and he started dating her friend. Ended up marrying her less than a year later, two weeks before basic training. He still felt guilty about it. May have been a bullshit story. I myself couldn't see why women would fight over him, but he sounded pretty sincere.