It was clear this was taken to mean that they were not having sex with another person. I bet all of them, or almost all of them, are regular masturbators. And of course, when masturbating they will be fantasising about having sex with another person. But the fantasy is better than the reality - or, at least, it often is. Of course, you would not get that from the General Household Survey.
Women are usually very coy about admitting that they masturbate.
I have found that even women who can talk quite openly about sex can clam up with embarrassment if you ask them about their masturbation practices. And the same is true of men. Masturbation still has something of a stigma attached to it, despite the fact that we all do it, even those of us who are in relationships. I have only had one long-term relationship. And it was not merely because at times we were apart.