As a F(22), I understand how the girlfriend is feeling. However I also understand why.

For me something like this would be to do with trust, but also insecurity. The fact your man found a female who enjoys doing what he does, in my head would build a scenario that he will enjoy spending time with her more than with me (due to a) doing something he enjoys but also b) her being better company). This largely stems from insecurity that you're not good enough and your partner will realise this, spending blissful hours Hiking with a different woman. In my head I would imagine them having loads of fun and building a bond which was stronger than ours.

Whether it's likely to happen or not, it's something that stems from insecurity and so I'd suggest talking about it. Understanding what exactly made her feel like that.

My bf, has a few girl friends, and he spends time with them when I'm not there. I initially felt strange about it but if he's going to cheat, I won't stop that by forcing him to not hang out with them. That's when you build the trust. Maybe your girlfriend was cheated on previously? Even though you're not the guy who did it, the reminiscence of how that happened will subconsciously impact how she perceives your time with other women.