LOL.. That was a nice solid NEG she hit you with there buddy, ouch!
Imagining myself in the same situation I would have laughed and told her with a big smile something like "look I dont really care who you have been banging in the past and i'm definitely not here play boyfriend with you now. I'm here to have a good time with someone I thought was a decent person I hope I wasnt wrong." And look at her as if she were little kid that just finished doing something wrong that they knew better not to do... but only briefly.
When a girl talks about her EXs she also opening the door to either admitting she slept with him making her a HO (well not really but for this arguments sake she is), or her saying she didnt bang him (making her very lame to hang with at all). Either way you have some cards you can play if done tactfully. Second, by saying that to you she is assuming you want a relationship with her. You can also just let her know shes not all that to be evaluating you in such a way in the first place. When girls talk to you about their old relationships either one of two things is going on. She's either friending you or trying to poop-test you. Either way I want her to remember i'm not good to talk to when it comes to that topic. Now is a good time to show a little agression, assertion and dominance. I ask all the most uncomfortable questions "Oh he sounds like a really nice guy did he ever expect anything in return for his kindness? I knew a guy that his girl kind of paid him with sex. That's so lame" Depending on her answers kinda take an authoritative high role in the conversation suggesting you have detected a moral defect in her personality. Flip the script bro, that's no Ace her sorry lil arse handed you! She would still be with him if their relationship was all gravy find out why they are not together now. Look I know these suggestions are not the "coolest" in town. But she's trying you and if you don't respond with getting a little more serious with her she'll walk all over you. Not only that but believe or not she testing you to see if you can put her in her place like her father would do. This needs to be the frame you adopt at that moment, very evaluatory, judgmental, maybe a little condescendant. One of the most important things about an aggressive strategy like this is that you always respect her, always keep the tone of your voice even. After all you are just having an unpleasasnt conversation, it doesnt need to end soon, nobody needs to win or loose. You are just directing this crappy interaction she has brought on you both.