What do I do about this uncomfortable feeling regarding my g/f's past?

Long story short, I recently found out that my girlfriend had sex with someone who is still a friend/close-male-friend of hers. This was before I started dating her (and even met her probably). They didn’t date or anything, just kinda hooked up. In the past, I was never bothered by my SO’s past, and even knew it to much greater detail with several X’s. But this time I can’t seem to shake this uncomfortable feeling. I don’t know which guy friend of hers it is, though I’m reasonably sure it’s one of 3–4 guys (most of whom I know). Nothing is wrong with any of them, they’re all nice guys. But I keep having recurring images of them having intercourse with my girlfriend in my head, which as you might imagine is problematic.

It’s almost worse because I don’t know which guy it was, so I’m having images of all of them. But then I wonder if it would be worse to know which guy it was for sure, cuz there’s no way I would ever look at him the same again.

I realize this shouldn’t matter at all. I know her past is her past, and for that matter mine is more extensive and I have several female friends that I have a history with (although I would note that I’m not really “friends” with anyone I’ve had sex with other than my g/f. Positive speaking terms and sometimes speaking to them yes, but I haven’t seen one in person in well over a year).

I find myself extremely uncomfortable and almost sickened by this… and I don’t really know what to do about it. I never wanted to be one of those guys who tried to tell his g/f who she could and couldn’t see (I always found that incredibly possessive and stupid), and even if she did I don’t know that that would get rid of the problem. It’s only been a week or two since I first discovered that a friend was among her X’s, do I just give it time? I need to figure something out, because I see several of these guys on a semi-regular basis, and quite likely will see all of them at least once in the next year or so.. and it’s going to be a problem if I keep thinking of them doing my girlfriend.