I disagree that the cited source has to specifically and exclusively treat the topic of hookup culture in order to be included.  A book on the 2008 American presidential election, for example, may focus primarily on John McCain and Barack Obama, but may also say something relevant about Joe Biden or Sarah Palin.  Just because they are minor topics in the source does not mean that source should be excluded in an article on the latter two individuals.  That is not what SYNTH is all about.