1. Dick Grayson (1940-1983) graduated to become Nightwing.
2. Jason Todd (1983-88) either was murdered or graduated to become Redwing.
3. Tim Drake (1988-2011) graduated to become Red Robin.
4. Damien Wayne (2011-).
This gives fan artists a lot of subjects for their fantasies.
The original Dick Grayson was tied and threatened constantly, becoming the traditional "Damsel in Distress" for Batman to burst in and rescue.
When I was a kid, you could get bubble gum cards with Robin being threatened in various ways.
More recent Robins aren't threatened quite as often.
But still, fan artists have been quite busy with depictions of Robin in bondage. There's a group on deviantart.com devoted to just pictures of Robin in Distress.
If you're going to show arousal or sexual activity, you have to make sure that Robin is over 18.
There are far fewer pictures of Batman himself being threatened, and those that exist tend to be humorous or bizarre, or both. Batman being basted?
I can't even speculate on why Robin is more popular as an object of bondage fantasy.

Even as an adult superhero.




















