Okay, so given your description of your gf and your relationship, you're having an irrational reaction. Here's the bit that jumped out at me:

That takes something from the "Past" and makes it a permanent fixture of the present.

Look at it this way: like all of us, your gf was a tiny baby when she was born, and I bet there are baby photos of her out there in her parents' photo albums. Yet you're not looking at her thinking "omg, this makes you being a baby a permanent fixture in the present, I can't date you!" That wouldn't be rational, would it? A record of the past does not take it out of the past. Your gf is who she is now, not a baby.

So why do you feel so differently about this issue? Because it's sexual. Your gf was a woman who had sexual activity with another person. You find this far more threatening than any of the other things she has been and done, and so you have these irrational thoughts about it.

In the comments you've talked about gut reaction and aversion. While there are many ways to get rid of irrational thoughts caused by anxiety, I think that in this case it's important to track down the real source of these thoughts - take a close look at the underlying beliefs you have that are causing these feelings.

I'm not able to guess as to what is going on with you, but you've talked here about needing to feel dominant and "another male" and another thing that stands out is you've also talked in comments about your gf's sexuality as something she "gives" to another person and her being used, rather than her participating in something.

So I guess you could start there - follow those down the rabbit hole for a few minutes. EG, what is being dominant? What would it mean if you weren't dominant? Is the thought of another "dominant male" a scary thought? Do you feel like she has opened the door allowing a stranger, a dominant male into your life somehow? (Don't answer these here - just think about them yourself)