I asked a straight friend this and he said...

"I have a video of a guy jacking off" "You don't see his face, just him jacking off"

The guy in the video also has a big cock and it takes him some time before he cums.

My friend said he does envy the guy and wants to have his cock. (No, not in him)

He also said he jacks off to it every once and a while. Not because it's a turn on but to have a buddy. That it's like two guys sitting on the couch jacking off to a porn.

Sometimes he will race the video and sometimes he will try to outlast the video.

I don't think this helps understand the straight guy big dick fetish... but I thought it was worth the post.