Sorry, having a hard time processing: what do you mean by “generally recognized cultural macrogroup with thick boundaries?” I think an example would really help me be clear on this.     

Any group whose is existence is culturally explicit, whose membership boundaries are reasonably clear, and whose culture is sufficiently all-pervasive that exogamy would be weird or stigmatized for any member.  Races, ethnicities, and religions are all likely candidates; at least in America, “college-educated people” is another. 

Point being, it’s pretty likely that someone of Fourth Quintile Hotness is together with someone else of Fourth Quintile Hotness – as compared to random distribution – but it’s probably much more likely that a white person is together with a white person, or that a college grad is together with a college grad. 

Exactly which categories matter in this way depends on where you are.  Hutu ancestry versus Tutsi ancestry has never been even slightly relevant in the US, but, uh…