Dealing with retroactive jealousy

[rajpoot]

The first thing she said to me after she saw him approaching us (right after, "Oh God it's XYZ") was, "Please be cool".

Thing is I'm not so certain about the 'challenged masculinity' angle. The very unreasonable reason that this is bothering me is that before this while I knew she had a past I never pictured her with anyone else. Now it's as if it's being rubbed in my face.
And while I knew I was relatively conservative, I never thought that it would ever be a problem for me, but now I find myself wishing that I was the only man she'd ever been with. I did not know this about myself before this incident and I would give a lot if I could simply change swap my point of view for more a reasonable and modern one. Because I know, that in this day and age wanting what I involuntarily seem to be wishing for is pretty much ridiculous.