Guys are turned on by visual stimulation. A woman masturbating is hot and quite a visual stimulus.

Put it this way...
Woman = Hot
Naked Woman = Really Hot
Naked Woman doing something Sexual = Really Really Really Hot

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Put me on wheels and I'll turn tricks.

Clever? Nah, I ran out of that years ago. But if you find this, let me know, k?
"The road goes ever on..." ~ Tolkien

In memory of my friend skip...
Go then, there are other worlds than these