"Plausibility of Denial."
With most porn pics one can claim innocence and that someone looks like them. There is always that chance that it's not your son... and then there are those cases--which I refer to as "Hi, Mom!" or in today's case, "Hi, Dad!"--where there is no deniability. They are looking headlong into the camera while doing something parents never want to imagine their child doing (at least on camera where the neighbors might see.)
"Dad, I'd like you to meet my daddy..."
What parent wouldn't want a picture of this moment?
"Remember all those times as a kid you thought I was playing with a pretend microphone...?"
This moment has a definite "Oh, shit" feel to it...
"I'm so excited for you to meet my new boyfriend!"
"I was going to call, but I had something in my mouth..."