You could turn it into a joke.
Her: are you sleeping with anyone else?
You: oh, hundreds! if not thousands!
You could dismiss it.
Her: are you sleeping with anyone else?
You: (casually) of course.
You could answer a slightly different question.
Her: are you sleeping with anyone else?
You: I have many friends.
A combination of above approaches that I quite like, the key words being "in particular":
Her: are you sleeping with anyone else?
You: there's no-one in particular at the moment.
The main thing here is your vibe, if you act guilty about it, she'll pounce.
A related issue is how much you tell her a priori. With my current gf I decided to tell her before sleeping with her, that I date other girls and intend to continue doing so. I told her the reason I was saying this was that if I didn't explain, then I would feel like I was tricking her into bed. (She's highly conservative). She got pretty schnitzel and left soon after. I regretted the decision heartily for a day or two, but was careful to treat her exactly the same, and she came back.
In another case a month or two later, I decided to just go ahead and sleep with her, and I put off the discussion to a few days later when she asked "am I your girlfriend now?". I replied something along the lines that, since it would be a long distance relationship I couldn't see that happening, but that even if it wasn't long distance I'm still dating and intend to keep doing so. This tends to operate as a boyfriend-disqualifier, it doesn't really harm your lover frame as far as I can see.
In both cases I used an example like "if you are at work and I am free, then I want to go out and have fun, so I'm gonna go talk to girls and see where it goes, I'm not gonna sit at home thinking I wish my girlfriend was here". I also said that I do not think people should form relationships too easily. And, that I do not believe in romantic love, because I think it is just our brains playing tricks on us, and that what we experience as romantic love is basically fear of loneliness in disguise.
Eventually, I usually disclose that I'm co-dependent and I basically say "if I wasn't dating other girls then I'd probably be acting needy and independent, calling you all the time and pressuring you to hang out, you wouldn't like that and neither would I"... this might be too much information, but I haven't found it harmful.
She tends to keep pressuring or bringing it up, either then and there, or a few days later... in that case I just expand on the above themes until she's bored of it.
Ray