I know that it's something that people should learn to let go of at an older age, but it's a hard thing to shake. And I'm sure this could go for both men and women.
Imagine that you meet someone that is really great overall. Good personality, in good shape, good looking, ambitious, a good age for you, no kids, and lots of fun. All around great.
Now imagine that said person has slept with or dated someone that just pisses you off, or at least
used to piss you off to no end.
The problem (for me) comes with imagining the sex between the two of them. The thought of someone I hate slamming into someone I really care about really grinds my gears.
It seems like the mature route would be to let it go if said person makes you happy, but holy hell. I am pretty envious of people that can get past this kind of thing.
Maybe you by circumstance are bound to have disdain for your SO's exes, regardless of whether or not they pissed you off before you met or started dating this person.
That's a good angle.
I might look at her different 
