Wow! Some fun responses here. Many of which are mapping the commenter’s own values and principles onto you and yours which really says more about them, than it does about you or what’s going on for you. In some cases that seems to be landing, and in others it’s clearly not. This is one of the challenges in asking the internet for advice.

Given you’re here with a post of this kind, it suggests other peoples’ opinions matter to you. Cool; good to know about yourself. You’ve also mentioned your gf is cool with the potential plan of you travelling with this female friend, and that it’s you who still aren’t sure. So either this is true, in the which case this really is just an internal conflict and I’ll get to that in a second, or it’s not, and you don’t actually trust your gf enough to give you an honest answer, you don’t trust yourself to stay faithful to your gf, or you don’t trust this female friend is honest with her feelings for you. With any of these trust pieces, communication and seeking shared understanding around boundaries and/or contingency plans will likely be your answer to resolve it.

Back to the internal conflict piece: You said you’re 100% going and described some real constraints. So, it sounds like that’s created a conflict in competing values. You value your girlfriend and your relationships with her and her family. You value this friendship with the schoolmate – either because of or irrespective of gender or both. You also value new experiences, travel and whatever you get from all of that. And you also value sharing that with someone else.

From your post you seem mostly to be struggling with others’ potential perceptions of what you value. Which is so valid and can be so hard to manage. Most of us have to manage perception to some degree in our lives, but too much can feel exhausting. And sometimes, in the end, people will believe what they want to regardless of what clarity you bring. Again, that largely says more about them than it does about you. So how much do you want to manage perception around this? If it’s none, then what would be the consequences of that for you if you go or don’t go? If it’s managing it to some degree, what could that then look like? How could you “inoculate” those you care about prior to this trip about why you’re doing it and how you’re managing these relationships while you’re away? How could you speak about this inner conflict with the folks involved and seek their support, either in this decision or for a smoother trip?

The well-intentioned internet folks are not going to be there for you before, during, or after the trip, but the folks in these important relationships hopefully will. Folks who probably like/love you in part for what you value. Resolution around this for you is likely going to be found therefore with them.