Hookup culture does not mean everyone hooks up. in fact, it means less people have sex.  


Pair bonding culture (marriage culture) =  women/men shamed for straying from marriage vows heavily. This means average guys get access to some average women for a family, because the average chick is not trying to fuck the top tier guy since he's married off already, and to do so would ruin her reputation.  


Hookup culture = 80/20 rule. 80% of all sex is being had by 20% of the best looking population of men. Women can punch up multiple attractiveness levels, because top tier guys are not getting married off, you are not being shamed for fucking him. This dries up sex for 80% of men, and increases competitiveness within that group for very average women.   


That is why hookup culture is shit, it's effectively putting us back into a tribal society where strong guys have a Harem and average dudes are left with the dregs.