math of survival

if a girl and a guy are attracted to each other 

but other humans prevent them from dating 

then guy will date some girl he is less attracted to 

and girl will date some guy she is less attracted to 

so guy reproduces with girl he is less attracted to 

and girl reproduces with guy she is less attracted to 

since there was less attraction there is less of a genetic match 

and their children suffer the consequences of that 

if this happens to a few guys and girls 

in every village and every city of a country all the time 

then that devolves humans in the population of that country 

and after a few generations the effects of that will be noticeable in that country 

before empires rose and fell 

later countries grew and collapsed 

visible throughout history