Porn websites don't use adsense to make money so that's rather irrelevant.
The trick is to find a happy medium, try and find a niché that has a pretty good CPC price but also one that isn't too saturated. Remember, it's better to be a big fish in a small pond than a small fish in a big pond.
The question you ask is really hard to answere. If others knew they either would not tell or they just dont know. You really have to think on this.. Try to think of something that people need or want... Any thing just want do..
I was in the same boat as you. Basically, just look around your room, the first thing you see, make a niche site about that. They only take an hour or two to build. On my first niche, I've made over $.50 on every click so far. And there isn't that much competition at all either!