Right now I don't know what you would gain with any further discussion of her past in that way. I've been married for 13 years and won't gain anything by discussing my wife's sexual past (I don't have one so there's nothing to discuss there). Personally, I would suggest just stopping any further discussion and tell her that you don't care about her past, but that she's with you now. (Unless she was hurt somehow, then that's worth discussing.)

As far as why you think one way and react the other, attachment to another person can be a pain. We over-inflate the other person's good qualities and then balance our happiness on them. Of course, everyone is flawed so when that perceived flaw comes out, our happiness with them becomes unbalanced. It's not them, but how we internalize our image of them.

Given more time with each other I think you'll be fine and be able to better deal with this situation.