Y'know, I hadn't thought of that. I suspect you're right....the entire train of thought as an effective prosecution strategy is ludicrous. ...
If the police had obtained this evidence in this way it probably wouldn't have helped their case. Surely someone on the jury would have said to themselves "They raped this boy to get evidence that he showed a dirty picture to his girlfriend...who didn't mind and had probably already seen the thing in real life anyway? WTF?" and then quietly exercised their option to nullify.
I feel pretty sure I would if I were on that jury.