Simply put, it's a fetish that you recently found and haven't accepted in your sexuality. Yet.
You: man the other day I was looking for things that tickle my fancy and I stumbled upon some lolicon...
Friend: and what happened?
You: before I knew, I was rock-hard. I feel so fucked up
Friend: as long as you don't molest real-life kids, I don't see the problem
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.