Like others have said, a "No" would be fine, but only in some circumstances.
What I don't like about their tone is that they pretend never to fib to girls.

I'm willing to bet that many of those hoity-toity above who said,
"Just say 'No, I don't have one,' because that's true and you should just tell the truth,"
have also said to girls, "Hey, let's come back to my place to watch that youtube video/have drinks/see my drawings/etc." Instead of, "Hey, why don't you come back to my place because I really want to fuck you."

It may depend on where you are with the girl, whether to simply respond directly and give it all away.
If you're deep in rapport/comfort, than a cocky line wouldn't be good.

Instead of a simple, "No." I might say, "Oh, wow. You're forward, but I like you. No, I'm single."
(but not in comfort).

I think this accomplishes much more than simply, "No."
What's this forum for again? What happened to this forum?

My above line cements the frame that she's chasing you.
And that you're comfortable with that frame.