I get these all the time. I wish I could write back, I'd say:

"Dude. If you had a clip of me jacking off to the porn I like to jack off to, and it was two frames with one showing the porn I like and the other showing how much I liked *that*, I'd give you money for it. That sounds hot.

And if you sent it around to my friends and whatnot, I imagine most of them would be like, "Woah. I had no idea Steve has such a big dick.""