Well the main thing I'd consider before breaking the news to my gf is to consider if she is a fan of competitive forms of fighting in general like boxing, wrestling, etc. If not then determine if she has ever felt in the slightest way aroused when beating another woman at anything and proving herself to be the better woman. U have to examine the nature of fighting and the greater meaning of what it embodies and find out if your lady is into things like general fight sports, intamacy of some sort with the same sex, competitive nature and things like that and present it to her based on the greater meaning of competition, sport, dominance, and then sexuality (don't start with the sexual arousal u get from catfights, make her see the deeper picture first and have her see it and love it for the same reasons u do. Don't just connect her eyes to it, connect her mind and sensual passion to it).
I know how scary it can be to present this to a spouse. My girl was indeed a wrestler in highschool, but it was just a sport to her and she didn't do it for the greater meaning of the sport she did it simply becuz it was the one sport she picked up as easy as walking and she had the perfect body for it. But at the same time she was always a total square when it came to our relationship. She was so jealous when I even said hello to another woman, and as far as sex she was so traditional and closed minded it made me sick. But one day I noticed she loved watching boxing and wresting, and she loved watching me train my body for martial arts, so I taught her the greater meaning of fight sports from the spiritual, competetive, and maetaphysical side and explained the sensual side as a biproduct of my love for fight between two well matched opponents, and not only did I get her to be cool with watching it wit me, we both trained together and started finding other couples right in the gym I trained with to have private sessions. not only did both our fighting skills take off but so did both of our sexual apetetites and experiences.
Good luck bro, just be cool and find a way to take what she likes and connect to what u love