Is the number of people she's dated high? And what's your relationship with this woman? Do you have feelings for her?
I can appreciate honesty, or whatever may appear to be genuine honesty, on part of a female, but I would rather it be on the basis of question answering, rather than just bringing it up randomly.
Men and women alike have to embrace the fact that humans will have a history. We're presented with experiences, and we are all-encompassing individuals who are fully capable of getting lost in a moment. Discussing relationship/sexual history is pretty much an implicit practice, but I understand how there can be some unpleasant feelings. The mindset we've been peppered with growing up always arises when trying to acknowledge if a certain number of anything, or a certain way of handling something, is the "right or wrong way".
Can't view women as lesser beings, not saying that you're doing that, but I would suggest trying to imagine something you would do in regards to partnerships/relationships, and then think that the opposite sex is fully capable of doing those same things. Asides from a few anatomical differences, we're all similar.