Well when it comes to this form, attraction is generally based on what you see considering you can't actually talk to them. So whether it is someone who is real you don't or whether an animated or CG representation of a character, there really isn't much difference on such a base level of attraction. Considering both either look, or in the latter part designed, to be sexy and attractive then it's hard to really make differences aside from the knowledge of creation and natural.
As for fantasizing about those you know and don't know, once again that would all come down to the individual person. On a base level, of course there is no difference. But it would depend how it would affect the individual person. Some might think it inappropriate to fantasize about those they have no personal interaction with or even the polar opposite with those you consider friends. Or some may even have personal issues with fantasizing over those that simply created to be sexy or "cute".
For me personally, there isn't much of a difference between the three. As stated, it is is a base level of attraction (visual appearance) and really there isn't much more involved in it. I don't think it affects any real-life instances, unless one becomes obsessed with fantasy rather than reality. But that would be a personal issues, not necessarily anything reprehensible of the actual process.
