You really want to know?
In case you haven't been on Earth for a while, porn is a fantasy (at least the good stuff is). The only reason this fantasy works is because we can put ourselves into the fantasy.
The beauty of the Stick Figures is that they're purely abstract. The lines are only present to indicate motion. Because they're so abstract, it's easy to imagine yourself doing exactly the sort of thing you're seeing. Heck, the action fires directly into your brain because there is nothing that slows it down (such as a face that you know isn't yours, or other distracting details). Zammo! It's not sex, but the abstraction combined with the direct link to the brain and the motions makes most of us think of sex. There are no details to distract us, there is only the motion, the motion, the neverending, hypnotic, repetitive motion. Hoo boy, your brain grabs a hold of that in no time and plays back the only sort of memory that it can associate: sex.
However, this society is accustomed to associating Stick Figures with children (although frankly, a lot of the art I've seen grown-ups do isn't much better -- they should have stayed at Stick Figures). So when we see a Stick Figure doing something as naughty as some of these figures, we make, in our minds, the association between children and what our brains are translating as sex. Whoopsie, is this your paradigm I'm boffing right now?
So you're repulsed by a child-sex association that your own mind is making, but you're attracted to the motions because, let's face it, this is the fastest and neatest bit of porn to ever jet straight to your hippocampus, isn't it?
I thought so.
Ready to hie thee to the Sticks, again?