This used to be a gigantic issue for me and has caused me untold grief in my life. I mean it would make me sick to my stomach.
Oddly in the past couple of years this has dissipated to a large degree (I never thought in a million years I'd ever be able to say that) and now I'm completely ignoring the issue altogether. I'm framing it that this was her in the past, that it's a new day now and she's with me.
It's hard to believe I'm even typing this because this issue would tear me up inside in the past. Now I have a strict "don't ask, don't tell" policy.
The worst thing a man can do is to ask how many she's slept with. It's bad because it opens up a Pandora's box inside your own mind and you will ruminate on it.
It's kind of like snooping in her email or other similar poking around. Invariably that stuff bites you in the ass. Invariably.
Same with asking about the past. You ask in the hope that you will find out that you have one of the .001% non-hores in existence today, but the odds of that are infinitesimally small in 2014. Almost all of them are hores, almost all of them have close to zero concept of loyalty, and almost all of them are delusional and easily emotionally swayed (by you but also by other men).
The media has destroyed them and made them sex-obsessed, and the best we can do is to create a new relationship, an new frame and to start training her early in the relationship.
She's been around. Don't ask questions that will cause you to fixate and ruminate on her past. Her past with other men is indeed disgusting to us men on a core level, because the women receives and absorbs the man, and carries him with her forever. That's why men are so repulsed by a woman's past and why women generally value a man's past. As with most things, it's the complete opposite.
Having typed all that I sure hope I continue to hold this view and that my old ways of thinking don't come crashing back out of the blue because like I said this was always an immense struggle for me.
The key is to assume the worst, because the worst is almost certainly true. She's got some mileage on her. Reset the odometer and look forward.