The sex appeal of a guy is apparently determined by the shape of his face.

Research carried out at the Natural History Museum found that, at puberty, the height of the upper face – the area between the lip and the brow – develops differently in men and women. Men have evolved short faces between the brow and upper lip, which makes their jaws appear larger, their cheeks more flared, and their eyebrows more pronounced.
Unlike other facial features, however, the difference “in upper facial height” cannot be explained in terms of men being bigger than women. Researchers believe men have evolved the features because they appear more masculine.