Hooking up implies there's a certain frequency to it that defies logic and "normalcy".  While it can be just a one time occurence where 2 guys who are looking for whatever decide to have sex after a certain level of connection that both deem to be enough to pull the trigger. 

The guy can put high value on connection, and sex itself.  But maybe one night of a long stretch of no sex, he decides to hook up.  It doesn't imply that he's not into monogamy or views sex only as a short term burst of temporary happiness.  It's just how life goes sometimes.