Yeah, unless you're in a monogamous relationship, who you sleep with is your business... "It's personal, I'm not a kiss and tell kind of guy". If it was me I wouldn't even have told her about there being other girls at all.. I would have just have ignored her question or joked about it ("yeah, of course I slept with like a zillion girls every day.. I mean, look at these abs!"

).. and if she had repeated it I would have negged her and teased her about it.

"Aren't you miss Insecure?"
If she does find out just tell her it's none of her business, and that's why you didn't tell her. You don't wanna know any details about HER sexual adventures either, right? I know I wouldn't at least.