Thugs like Gary are the jizz stain on the black T-shirt that is our national sport.
I've given some thought to this (and my head hurts) and think it wanders into simile teritory.
Obviously to be a simile it has to comparable, so if you'd said
"Thugs like Gary are the jizz stain on our national sport"
Then that would definately be a metaphor you are mixing together 2 different objects to describe one as the other.
By adding in the "on the black t-shirt that is our" you make it comparable you seperate out the to objects in the statement ('gary and football' and 'jizz stain and black t-shirt'), and basically make it a pleasent sounding rewording of "Gary is like a jizz stain on a black T-shirt"
Thinking about it though it's more than possible that I've just spouted a load of poo and you can dissect any metaphor in that way. I gave it a good shot though and for that I'm proud. I wonder why I only got a C in my GCSE English.