<human>: Give me the gist of the text in a nutshell

Disclaimer: I'm seventeen and stupid; spelling, evidence, examples, etc are all likely awful and not well described but i'm looking for some feedback from some non-biased people. All feedback is highly welcomed, please and thanks!

For about two months now my girlfriend has been asking me some very personal questions about hooking up with other people, most of which sounded like general curiosity with the opposite sex. Through a few discussions i found out that she highly considered the possibility of hooking up with any other girl but told me she wouldn't because she was with me and preferred me over another girl which was her best friend, fictitiously named Amber.

Fast forward to last night when five of us (including the three of us) had a bit of a party and had a few drinks. ('OoO underage drinking' yeah i get it, scold me another time). Very late into the night it came out that I was the only thing holding my girlfriend back from having a relationship with Amber, and Amber reciprocated the intentions of being together. I was shocked to be honest, it seemed like i was holding them back from something and it hurt to know.

My girlfriend then took me aside and told me that a while ago Amber and her had made-out at a past party while drunk, but my girlfriend, after a few minutes stopped and told Amber she wasn't comfortable doing so because she had a boyfriend (myself).

If i'm being completely honest i'm pretty hurt, this girl means the world to me but i cannot see us being together anymore knowing that she wants to be with me only slightly more than Amber.

Through writing this, i realize i might just be over thinking things, but i really need some non-biased opinions on what i should do. i can't really describe the situation without writing an entire novel , so i summarized the main points as best i could, (again i'm seventeen and stupid give me a break). I feel uncomfortable being with someone who wants somebody else, and it makes it even harder considering i genuinely care about my girlfriend.
<bot>: My girlfriend of two years wants to experiment  with the opposite gender, and it makes me uncomfortable. What should i do?