Hey man,
You cannot say you don't, that would make you either a poor sexless chap, or a liar. You cannot say you do - at least directly - because what the hell are you doing with her then.
Keep her guessing. Either you have an enigmatic half answer - "well, I'm kind of an in-between period of my life...". Or you give a cocky, enormous answer "I have two girls. One in the morning for breakfast, the other late at night just before sleep" with a huge playful, teasing tone. She can't take you seriously, but you keep her guessing. And if she comes back, you hold your position firm.
Other convo I used before:
Her: How many ...
Me: (interrupting her) any question starting with how many is a wrong one
I've seen two types of girls. Those who more or less suspect, but don't want to know for sure. These ones avoid the questions, so no need to shove it to their face. And those who suspect, but their curiosity (about how many) is too high. These ones will insist, again and again and again. Up to you, but in this case I just tell them with no shame.
Cheers,
Seppuku
