I agree with the posts suggesting that it is not the content of the fantasy that is important here, but the feelings under it. This sounds to me like a fantasy of testing limits - he imagines a sub who is so devoted to him that she would be willing to beg for something that, in general, is pretty taboo. It's not clear whether he wants her to want it, also, or if part of the pleasure would be in her acting like she wants it when really she doesn't, just to please him.