You can see who other people are attractive (either famous or around you), and infer from there.
OK, that makes sense. Perhaps I can't apply the second criterion because I'm too uninterested in who other people want to have sex with: see my reply to rchaud.
But I seem to have a problem with the first criterion, too. Lack of imagination, perhaps? I can't/don't think about having sex with someone unless I've already done it or have come quite close? Or perhaps I'm just non-visual? (I'm a bit face-blind.) So if you were to show me a selection of women of child-bearing age with no obvious signs of ill-health and ask me which I'd most like to have sex with then my reply would be either (1) none of them, because none of them is my wife, dur, or (2), in a different era or parallel universe, all of them, in any order, and then I can perhaps tell you which I prefer, because, as far as I'm concerned, a large proportion of the female population tie for first place when it comes to looks. (Though only a tiny proportion of them have a really nice voice: I'd probably find it a lot easier to pick a favourite voice from a set of recordings.)
So I hope I'm never a subject in a psychology experiment that requires me to rate how "attractive" people are from a picture. In most cases I just can't do it.