Actually as a male I'd say if an unattractive woman hits on me my just staring at my junk or my ass, I'd be flattered.
Only time id feel bad by it is if they are getting handsy or insisting they get something from me I dont want to give. But if its simply compliments and staring, and if they respect whatever boundaries I put up, then I would be nothing but overjoyed whether the woman is attractive or not.
That said I do agree, asking would probably be met with weird reactions and unfavorable responses for both sexes just because its just a weird thing to ask. But from the male perspective since I think most males would love someone "checking them out" im not sure there is a need to ask or the awkwardness that comes. Women on the other hand wouldnt be so enthused with the staring so its a differnet equation.