But - again, respectfully - you are looking at this through your lenses. And while it's cool for you to do that for your own wellbeing, and it is cool to explain it as "that's just the way I am", remember she's looking at this through HER lenses, and, well, "that's just the way she is". You have no idea if this is a "hot stove" for her (meaning, she's touched this stove before and been burned).
I'm seeing someone now who is - objectively, not just my taste - very beautiful. She has actually won a beauty pageant beautiful. So she has many male friends. And I am generally pretty cool with all of that; I am secure in my masculinity, and even though I'm not as good looking a man as she is a woman, I have my upsides, and I know what I bring to the table. But I have a "history" with "friends", including the ending of my marriage by virtue of a "oh, but we're just FRIENDS" relationship. An "Oh, but we just WORK together" relationship. And a "Oh, he was just JOKING with that reference" relationship.
Things can get awkward whether too much or too little history has been divulged. To offer a view from a perspective perhaps similar to the one of your friend's girlfriend, sometimes these things bother me, and even though it's only a bit of a bother, the smaller the stone in your shoe, the more it hurts while you walk.
My boyfriend hasn't really divulged any of his history to me. All I know are the small things - out of the girlfriend's he's had, one or more lived in my city (we're long distance), there was one who broke his heart, and at the time I was pursuing him, he was pursuing other girls he liked, but decided to give "us" a go. I very carefully explained my "history" (not much to tell, one real long relationship and that's it) to him, considering the fact that my ex-boyfriend remained a friend of mine and we are in the same circles still, and I thought I should avoid any misunderstandings. I was very clear about the fact that our relationship died because we weren't attracted to each other at all anymore and couldn't function romantically at all - that it's done, dead, gonners forever.
But he hasn't extended this courtesy to me, and while I am a reasonable, liberal person, it bothers me that, considering our circles of friends and acquiantances overlap, I might have as well already had coffee with one or more of his exes or old flames without knowing it. I wouldn't have a problem with it if I knew who they were, but if I was casually mentioning my boyfriend while hanging around waiting for an event and a girl next to me said "oh yeah, he's my ex", I would be taken aback. This extending to the fact that he's had old flames and what-could-have-beens I may already know about is just not a good place to be, even though I've made myself at least appear very cool with it. I don't even know what I'd do if one of them presented him with a sex toy in front of me
clearly I wouldn't think she was trying to show me up, and I am very familiar with the fact that you can be friends with an ex, but it would still feel awkward.
So I'm just in this intersection where I do feel insecure, but I also feel insecure for being insecure. I know everyone has history and that worrying about it is extremely uncool. But I also know there are probably people out there my boyfriend would still have feelings for if their paths crossed again, even if he wouldn't act on it. And that there is a chance that he's had feelings of the magnitude he'll never have for me. While people have so reasonably said that everyone's insecurities are their own business, and that relationships you don't have utmost confidence in should be terminated, in real life it's not so clear cut.
While in all relationships we can handle being a part of a larger circle, romantically we can't. It's not a fear of being cheated on, it's more of a fear that you are the second best, or not the one and only, and that there will be other people after you. Maybe for her you seem like one of those other people. But you did all you could to assure you you're not, the rest is up to her.