GF doesn't like me hanging out with female coworkers

I know this sort of thing has been posted before, and I've read several threads about people's girlfriends taking issue with their boyfriend having female friends. Most comments seemed to side with the boyfriends, some even saying that men having female friends is a big green flag for them, which I am compelled to agree with. I don't post much, so I apologize if this has been posted to oblivion and is redundant, but I guess I was hoping to get some feedback on the specifics of my situation.

For context and setup, I've known this girl I was dating for several years, but we only just recently started talking more seriously with each other, with the intent of starting a relationship. I am an American living in the US, and she is German living in Germany, so our entire time of knowing each other has been long distance. Not ideal for starting something serious, I know, but I try not to rule out the possibility of something good coming from unideal circumstances.

For further context, I am a cargo airline pilot, so I travel frequently and to very many different parts of the world. I know pilots have a certain reputation of meeting women in many different locations, and that is not me. I do not actively seek women for sex, or even friendship, while I'm on the road or during layovers.

Part of my job involves flying with different crewmembers for each leg of my monthly pattern. This means that for the most part, I am flying with a new crew every time I get into the plane. Some pilots are men, some are women. Makes no difference to me. There are some places that are "hot spots", where there are several crews coming and going, stuck because of delays, layovers, etc. This coupled with keeping in touch with classmates I was in training with makes the point: sometimes we're bound to run into people we know.

Now, one of my flights was with a female pilot, a very charming, polite, and friendly girl. We had some nice chats about common interests, life experiences, life goals, etc. which is nothing unusual for two people who are trying to be friendly towards each other. Nothing against this girl, but I did not have any kind of physical or romantic feelings towards this girl at all, mainly because I was being more serious with the German girl, but also because it just wasn't there for me. I also did not detect any feelings or attraction from her towards me, so as far as I could tell, we were just two normal friendly people being normal friendly people towards each other. At some point in our chats, I had mentioned that I was seeing someone as part of the topic we were talking about (I didn't just come right out and say it, it had something to do with what we were talking about at the time) and that didn't elicit any kind of response from her, not that one was warranted. We friended each other on facebook and that was that.

Fast forward a few weeks, and I am now sitting in Anchorage, Alaska on call, waiting to be called for a flight if I'm needed. Anchorage is one of these locations where there are always pilots coming and going, and have run into several people I know from past experiences, and this same girl was on a layover for a few days. She reached out and asked if I wanted to hang out, to which I said sure. We went for a nice walk at a local park, swapped some stories, enjoyed the brisk air and nice views, and then went our separate ways. The next day, the GF asked what I did that day, so I told her, and let's just say she fervently expressed her disapproval of my actions.

I like to think I have a pretty calm demeanor, because I've learned over the years that acting and saying things purely based on emotion usually doesn't go well. So for the past two days, I have been trying to talk with her to understand exactly what the issue was, and my approach to this is to ask her to clarify exactly what her issue is, and why she is feeling the way she feels. For the most part, she would answer these questions by just reiterating her feelings, or by saying that it's a "no-go" for her. To me, this doesn't really answer the question, it just tells me something I already know: she doesn't like it. After continuing to try and get to the bottom of it, she would say things like "I don't do that sort of thing with male coworkers, except ones that I've been friends with for several years, so I expect my partner to do the same" or "there's no point in trying to explain further because you just will not understand."

She has expressed to me several times in the past that she has trust issues, mainly due from a previous traumatic relationship, and from just men in general. I believe this lack of trust to be the main crux of the problem here, but I also don't believe she ever acknowledged it enough to consider that maybe she was being a little too harsh in her conclusions. My main issue with these events is that I don't feel like she ever gave any real consideration to me, my opinions, or my capacity to be friends with a female and have it be nothing more than that. I believe this to be because she simply doesn't trust me enough to be a man and not constantly feel the need for sex from every woman I meet.

I apologize for how long this has gone on, but I wanted to try and paint an accurate picture so that I could then ask my questions. Was I in the wrong? Should I not be friendly to women if they are friendly to me? There was no flirting of any kind from either side, so if this was really just two people being kind to each other, should I have not gone to hang out with her? I think this relationship has sailed, but I am looking for feedback and lessons to learn here. I'm curious if I really did something wrong, or if I should expect all women to share her views that forming new friendships with female coworkers that I happen to cross paths with on occasion is really such a terrible thing to do. Thank you in advance for any help and insight.

TL;DR girlfriend doesn't like that I hung out with a female coworker friend, even though there was nothing romantic about it.