If you do want to be exclusive with her:
When she asks you, just be a bit shocked and say, "I don't know... I'm going to need some time to think about this." Then a couple of days later, call her up or go on another date and tell her, "I've been thinking about what you said about being exclusive. I guess we can try it for awhile and see how it goes, just don't get any ideas or think it's a permanent thing."
If you don't:
Again be a bit shocked and say, "I really don't think we've reached that point quite yet. Let's just keep dating for awhile and see where it leads before we decide to become exclusive."
(Be careful before rejecting though! Think about if she really is what you're looking for. It's always okay to do the "I need time to think about this" and then a couple days later say you want to keep dating non-exclusively if you really do need some time to think. It's also okay to accept her offer, then end it later if you decide it isn't working for you.)
magickarl said:
Personally, I think you would be happiest telling the girl that you are exclusive to her, and then getting tang on the side. An important DJing skill is mastering the juggle effectively.
No. Absolutely not. This is wrong. There is no excuse for lying to a girl about this... it's called cheating. Think of how you would like it if a girl you were devoting all of your romantic energy to exclusively were secretly dating/having sex with other guys on the side.