My girlfriend (F37) has a romantic relationship with her best friend (F37).

I've been in a relationship with my girlfriend for a year. Overall, we're very compatible and care for each other deeply; however, I've started to feel uncomfortable with her close friendship with her best friend, which they've maintained for 15 years. At the beginning of our relationship, my girlfriend mentioned her best friend, who now lives abroad, and the strong bond they share. As time has gone by, I've noticed some behaviors that have raised concerns for me. They've had a few threesomes together in the past, and there were instances of physical intimacy between them, including kissing and touching, which I initially dismissed as part of her past. A few months in, I discovered that they were exchanging nudes, and I expressed my discomfort with this. My girlfriend agreed to stop sharing those photos. However, I soon became aware of other behaviors that bothered me, like her friend often mentioning how she misses their shared intimacy and the fact that they exchange clothes to feel connected. It made me question the nature of their relationship, especially when her friend showed signs of jealousy over new relationships. The situation escalated when I learned they had a pact where her friend said she would "marry" my girlfriend if she couldn’t move to her country. This really upset me, as it felt like it hinted at something beyond friendship. I confronted my girlfriend, expressing that all of this makes their connection seem romantic, regardless of the fact that her friend is a woman. She insisted that their relationship is separate from ours and that she would never leave me for her friend, but I can't shake the feeling that there’s something more to it. My girlfriend does have a slight romantic inclination towards women, which complicates things further. I've noticed times when she seems to prioritize her best friend over me in decisions that affect our relationship. An instance that stood out was when they planned a trip together, and her friend joked about finding someone for my girlfriend to be with during the trip. Even though my girlfriend declined, it felt out of place for a friendship. Despite my girlfriend's reassurances that their relationship doesn’t impact ours, I feel like the dynamics between them invade our relationship space. How should I approach this issue? To me, this is a boundary that shouldn’t be crossed. **tl;dr:** My girlfriend has a very close friendship with a woman that displays signs of a romantic dynamic, making me uncomfortable.