I think that the whole, 'celebrities are real people' is kinda BS. I know that in real life, there is a woman named Jessica Biel, that lives in West Hollywood and is married to pop superstar Justin Timberlake (or was? are they still together?).
But the only thing that I know Jessica Biel for is the fact that she makes a living pretending to be other people. Usually really hot people, but that's neither here nor there. I only know her in the context of the fact that she lends her image to portraying fictitious characters. Therefore, when I fantasize about celebrities, it's not about them, but about the image that they use in the course of their professional life. So I don't think that there's somehow some kind of extra ethical problem with using celebrities (any more than there are usual ethical problems with this fetish).