That is not always the case. Felonies are not always violent. Some laws are drafted so broadly that widely different behavior is made illegal. Lewd behavior is a felony in some states. Lewd behavior can run all the way from relieving yourself in an inappropriate place, to social protest, to sun bathing, to the acts of a really perverted mind.
Pornography can be totally disgusting and is a felony in many places.
On the other hand what is pornographic changes. The term has been applied to classical art such as all those naked Greek statues. It has also been applied to parents who took naked pictures of their children as babies in typical baby behavior. In many states gambling is a felony. Procuring an abortion used to be a felony.