I think there are really only three ways to respond to shit tests:
1. Agree and amplify. You say stuff like this with a joking manner. For example, next time, for the first question, do not get defensive and instead amplify and exaggerate with something like yes, I bang girls every day Monday thru Friday but rest up on the weekends. or something like Well, practice makes perfect. So no days off. Or since you didn't do this on the first reply, you could say yes, you should stop talking to every other guy in your life including your father. She must also wear a veil and walk two spaces behind you at all times and refer to you as master in public and so on. The point here is make her claim ridiculous.
2. Be upfront. This is basically Evan Wilson's response. Another way to do this is just say I don't want a girlfriend. I like being single. You can even throw in some philosophical shit about how you were burned in the past and don't want to go through that again. Or at your age or at this point in your life, you are too young or simply do not want monogamy. You are just being honest about your intentions and don't want to agree to something you won't be able to follow through on. You could also throw in the part that you are attracted to her and that in time maybe this could grow into something more. But this last part is kind of a weasel way of doing things if it isn't true.
3. Ignore and deflect. This is basically Hank Moody's response. Sometimes the best response is to not respond to it at all as every question doesn't merit a response.
All of these responses will give the response that you are ok with her seeing other people. And this should be fine as you are laying down the law so long as you really will be ok with her seeing other people. You will only get into real trouble if you decide at some point you want monogamy and don't tell her. I don't think there is a response that you can give that will keep her truly happy. She will ultimately monkey branch when she finds another guy who will be monogamous if that is what she is truly after. But that may be in some time. I think #2 is probably your best option at this point.
Or you could just go the break up route. Here, is a humorous one for you:
https://www.youtube.com/watch?v=Gab7rrzOr2I